BanRussiaAtIOI's blog

Codeforces Round #571 (Div. 2)

By BanRussiaAtIOI, 4 years ago, translation, In English

Hi everyone!

The rated Codeforces Round #571 (Div. 2) will take place on Jun/28/2019 11:20 (Moscow time). This round is based on the Kremenchuk Summer Programming Cup 2019.

The contest is prepared by Dalgerok, Karasick, stanislav.bezkorovainyi, antontrygubO_o, danya.smelskiy, and me.

You will have 2 hours and 15 minutes to solve 6 problems.

Good luck!

We apologize for the issue that happened with the problem B. The round is unrated. If you want to read my emotional reply about it, please read this.

  • Vote: I like it
  • -802
  • Vote: I do not like it

Comments Comments (495)
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4 years ago, # |
  Vote: I like it +53 Vote: I do not like it

shortest blog for CodeForces round XD

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    4 years ago, # ^ |
      Vote: I like it +4 Vote: I do not like it

    I mean, you know the saying: Quality over quantity. xD

    Anyways, good luck to all participants!

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4 years ago, # |
← Rev. 2   Vote: I like it -20 Vote: I do not like it

there is one more thing you wanna say

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4 years ago, # |
  Vote: I like it +51 Vote: I do not like it

I hope the statement as short as the blog :)

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    4 years ago, # ^ |
    ← Rev. 2   Vote: I like it +15 Vote: I do not like it

    Why hope? You won't even participate as far as I know.

    UPD: He is my friend, and I asked him about that, he said he won't participate in the whole contest.

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      4 years ago, # ^ |
        Vote: I like it -69 Vote: I do not like it

      I'm sorry but his name is Black_Ghost, not "my friend".

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      4 years ago, # ^ |
        Vote: I like it +11 Vote: I do not like it

      I guess he knew about problem B all along. He's happy now.

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4 years ago, # |
← Rev. 2   Vote: I like it 0 Vote: I do not like it

I got it all wrong, it was not you

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4 years ago, # |
  Vote: I like it -17 Vote: I do not like it

It was wonderful to set the contest for 2 hours and 15 minutes instead of 2 hours.

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    4 years ago, # ^ |
      Vote: I like it -22 Vote: I do not like it

    It will not affect much!

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      4 years ago, # ^ |
        Vote: I like it +92 Vote: I do not like it

      It will not affect you and the people like me Who just solve 1 or 2 questions . :D

      And then they are done because they don't try to solve further .

      For others it matters a lot .

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4 years ago, # |
  Vote: I like it +42 Vote: I do not like it

Karasick vs karasek

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4 years ago, # |
  Vote: I like it +105 Vote: I do not like it

You forgot to thank MikeMirzayanov for codeforces and polygon platform.

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4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

What do they mean when it is said that the round is based on a certain competition? In this case the "Kremenchuk Summer Programming Cup 2019".

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    4 years ago, # ^ |
      Vote: I like it +3 Vote: I do not like it

    it means there was hosted (or there will be hosted) a local competition and they decided to use its problems at cf round

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      4 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      But wouldn't that mean that some coders might have already solved these problems?

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        4 years ago, # ^ |
          Vote: I like it +1 Vote: I do not like it

        They are asked not to participate

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        4 years ago, # ^ |
          Vote: I like it +59 Vote: I do not like it

        This round is during the official competition. Participants have no way to take part in this CF round.

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4 years ago, # |
  Vote: I like it +122 Vote: I do not like it

This day is my birthday, I hope I will become yellow

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    4 years ago, # ^ |
      Vote: I like it +31 Vote: I do not like it

    I think you will become Red instead of Yellow because it is your birthday #GPL

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    4 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Happy birthday in advance!

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    4 years ago, # ^ |
    ← Rev. 3   Vote: I like it -41 Vote: I do not like it

    Btw, MikeMirzayanov, ban him. This guy is clearly using multiple accounts:

    https://codeforces.com/profile/nomapunk

    https://codeforces.com/profile/47th-Draganov

    https://codeforces.com/profile/ProgrammerSasha

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      4 years ago, # ^ |
      ← Rev. 3   Vote: I like it -11 Vote: I do not like it

      It doesn't matter.

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        4 years ago, # ^ |
          Vote: I like it -16 Vote: I do not like it

        "It doesn't any matter,OK?" is grammatically incorrect. This is an example of a fact that actually does not matter, because there are few native English speakers on Codforces and almost none English is perfect here (including mine).

        On the contrary, if a person has two fakes from which he writes contests, he indeed shows disrespect for the competitive programming community. You may say, that i am writing from a fake account and show disrespect for the community too. This is a valid point, but i do not plan to write contests from this account and, therefore, won`t affect the rating distribution and anything related with contests themselves. Secondly, i just wanted to write this using that exact handle.

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      4 years ago, # ^ |
        Vote: I like it +8 Vote: I do not like it

      Im curious over how you deducted that this person has multiple accounts.

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    4 years ago, # ^ |
    ← Rev. 2   Vote: I like it +4 Vote: I do not like it

    By the way, how do we know you are not a contribution s**t? Anyone can write this and gets tons of upvotes, and, therefore, contribution. This comment does not provide no useful information, no joke or funny meme, it is not a question. It does not provide anything! Although i realized that i can not understand cf community properly, i still have no idea why this got so many upvotes. So, lets conduct an experiment. I will write a similar comment and will see, whether it will get upvoted or not (i bet it won`t).

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      4 years ago, # ^ |
        Vote: I like it +1 Vote: I do not like it

      Shitposting is a perfectly valid mode of expression online

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    4 years ago, # ^ |
      Vote: I like it +5 Vote: I do not like it

    ohho, many many happy returns of the day, Happy Birthday in advance. :)

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    4 years ago, # ^ |
    ← Rev. 2   Vote: I like it +29 Vote: I do not like it

    I hope to become yellow as well because it's nomapunk's birthday

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    4 years ago, # ^ |
      Vote: I like it +1 Vote: I do not like it

    Good luck.Happy birthday!

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    4 years ago, # ^ |
      Vote: I like it +6 Vote: I do not like it

    Happy Birthday!!! I wish you that each problem you solve would be as interesting as this round`s B!

    All the best!

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4 years ago, # |
  Vote: I like it +63 Vote: I do not like it

The time is unusual.And it is friendly to Chinese！

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    4 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    so strange I feel. Although I think in here, the words Rabbit said seems emotional and rude. But, both sides’ words are really harmful to this fair community and very disrespectful. I don’t know how everybody who looked at these comments, also upvote or downvote to both them thinks. At least I regards these behaviors are unreasonable.

    Ok, I’m just using my right to express how my feel, and also, take my downvote. Mmaybe a downvote can’t represent or change anything, there may be a lot of people on the other side. So what, that’s my right¯_(ツ)_/¯

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4 years ago, # |
  Vote: I like it -10 Vote: I do not like it

Do i need to register or can i directly participate..I don't see register button..

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    4 years ago, # ^ |
      Vote: I like it +2 Vote: I do not like it

    contest has ended. before contest you will see a register button

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4 years ago, # |
  Vote: I like it -12 Vote: I do not like it

May I become an orange color after this round?

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4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

New to codeforces and been reading this a lot "based on......". What does thos actually mean?
Are the questions same, similar or based on something taught there??

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4 years ago, # |
  Vote: I like it +5 Vote: I do not like it

Hi i'm very new to CP. I'm not sure if this is the right place to ask my question but sorry :-) I've joined CF recently. In my profile it is showing unrated. What should do to get rated.

Please help me I want to improve. Thanks to all in advance

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4 years ago, # |
  Vote: I like it +3 Vote: I do not like it

Can someone give more information about the "Kremenchuk Summer Programming Cup 2019"?

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4 years ago, # |
  Vote: I like it +18 Vote: I do not like it

It's a nice time for Chinese CFer

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4 years ago, # |
  Vote: I like it -14 Vote: I do not like it

Kek, authors have 3 IMO medals. Why not 3 golds, antontrygubO_o? :trollface:

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4 years ago, # |
  Vote: I like it -35 Vote: I do not like it

This day is my birthday, I hope I will become cyan.

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4 years ago, # |
  Vote: I like it -9 Vote: I do not like it

good luck !

hope good color for all ♥

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4 years ago, # |
  Vote: I like it +13 Vote: I do not like it

As a Chinese I think the time will be good for us.
However, I'm in Russia now and thus the time is not that friendly for me to participate lol.

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4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

I was introduce with programming just 1.5 year ago. I have created my codeforces acount then. But still i don't know about the rating system. Anyone please help me to know about the rating system.

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4 years ago, # |
  Vote: I like it +1 Vote: I do not like it

It feels to be good on Codeforces!

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4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Contest time is suitable for Chinese.Good luck for everybody.

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4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Hi everyone I want to solve questions on graphs and trees but i don't know basics about tree and graph.Please anyone help me from where i can learn basics and after that i can solve questions on graph and tree. Thanks in advance.

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4 years ago, # |
  Vote: I like it +59 Vote: I do not like it

Why contest delayed by 15 minutes? It's definitely a bad culture of Codeforces!

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    4 years ago, # ^ |
    ← Rev. 2   Vote: I like it 0 Vote: I do not like it

    I think the purpose is to gain more participants, but since the start time is different from the usual contest, the fact that there are fewer participants is not strange. Anyway, it's time to read comments. :)

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4 years ago, # |
  Vote: I like it +4 Vote: I do not like it

Delayed by 15 minutes

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4 years ago, # |
  Vote: I like it +28 Vote: I do not like it

Sorry about the delay. We have some internet issues onsite.

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4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Will there be a 12 hour period of hacks for this round after the contest?

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4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

that internet issue again

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4 years ago, # |
  Vote: I like it +3 Vote: I do not like it

Seems that, codeforces is trying to maintain it's dignity(as well as uniqueness) by delaying contests nowadays.

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4 years ago, # |
  Vote: I like it +1 Vote: I do not like it

![ ]( ) we are still waiting

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4 years ago, # |
  Vote: I like it -11 Vote: I do not like it

looks like that code forces is testing it's users's patience !!

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4 years ago, # |
  Vote: I like it +3 Vote: I do not like it

yes 15m delay ...…. Thx from Bangladesh !!! its Friday …. ( salat time if the contest start before time)

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4 years ago, # |
  Vote: I like it +1 Vote: I do not like it

So real!!!

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4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Score distribution?

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4 years ago, # |
  Vote: I like it -11 Vote: I do not like it

I had registered for the contest . but during contest it showed i didn't register for the contest and thus couldn't submit solution . Please , Codeforces solve these type of problems soon.

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4 years ago, # |
  Vote: I like it +11 Vote: I do not like it

I think the sole purpose of this contest is drop my rating, getting easiest A and level goes high up from B.

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    4 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    it's true about me too ; i'm exactly like you now !

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    4 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    ur lucky,round became unrated

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    4 years ago, # ^ |
    ← Rev. 2   Vote: I like it 0 Vote: I do not like it

    Same here :)
    Had some network issues and couldn't submit 1st problem with good speed.
    But it's unrated now :D
    We are lucky.

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4 years ago, # |
  Vote: I like it +2 Vote: I do not like it

the contest is unrated now :(

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4 years ago, # |
  Vote: I like it +176 Vote: I do not like it

The problem setter of B should stop creating problems.

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    4 years ago, # ^ |
      Vote: I like it +4 Vote: I do not like it

    how to solve c , can u tell me

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      4 years ago, # ^ |
      ← Rev. 2   Vote: I like it +23 Vote: I do not like it

      let sum(l, r) be the number of "1" appears in range[l, r] in the string a.

      and the answer is the number of range [l, r] that (sum(l, r) % 2) == (the number of "1" in string b % 2). and of course (r — l + 1) = |b|.

      you can see the code of my another account Dilute_ :P

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        4 years ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        (sum(l, r) % 2) == (the number of "1" in string b)

        Can you clarify the above line please? Because LHS would always be 0 or 1

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          4 years ago, # ^ |
            Vote: I like it +2 Vote: I do not like it

          I believe Dilute meant sum(l, r)%2 == (no of "1" in string b)%2

          because say there are x no of "1" in substring of a and y no of "1" in b and z be the no of "1" that are in same position in that substring and b

          then required number of different will be x+y-2z

          so if x + y is even, that substring is counted in final answer

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            4 years ago, # ^ |
              Vote: I like it 0 Vote: I do not like it

            Thank you and How you come up with this?! Do you have some similar problems on cf?

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            4 years ago, # ^ |
              Vote: I like it 0 Vote: I do not like it

            how please explain or if u have link of similar problem share it

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              4 years ago, # ^ |
                Vote: I like it +5 Vote: I do not like it

              Proof for problem C. BanazadehAria

              Let there be $$$x$$$ number of 1s in substring $$$c$$$ of given $$$a$$$ and $$$y$$$ number of 1s in substring $$$b$$$.

              Let $$$t$$$ be the number of 1s that match[overlap] in both the strings. So exactly $$$x-t$$$ number of 1s do not match in $$$c$$$ and exactly $$$y-t$$$ number of 1s do not match in $$$b$$$.

              Total mismatches = $$$x+y-2t$$$, we need to find whether this is even. $$$2t$$$ is even, so it's sufficient to check whether $$$x+y$$$ is even.

              56238050

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            4 years ago, # ^ |
              Vote: I like it +9 Vote: I do not like it

            Well.. It was my fault and I have fixed it now :D

            I just wrote the comment without checking carefully, but I just meant it modulo 2 QwQ

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        4 years ago, # ^ |
        ← Rev. 2   Vote: I like it 0 Vote: I do not like it

        You do know that you are not supposed to have two accounts, right?

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    4 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    LOL! But why? Is it because it was very difficult in comparison to the points assigned to it or because the judge's solution was wrong.

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    4 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Exactly. I didn't like that problem either.

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    4 years ago, # ^ |
      Vote: I like it +89 Vote: I do not like it

    I'm really interested in why so much people solved B...

    and their solution are all the same as the writer's wrong one...

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      4 years ago, # ^ |
        Vote: I like it +5 Vote: I do not like it

      Because a lot of people follow their intuitions blindly :-)

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4 years ago, # |
  Vote: I like it +10 Vote: I do not like it

anyway, will there be a tutorial for the B ?

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    4 years ago, # ^ |
    ← Rev. 2   Vote: I like it +8 Vote: I do not like it

    I think the answer is (n+1)*(m+1)/6, but I didn't prove it yet.

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      4 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      its wrong

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        4 years ago, # ^ |
          Vote: I like it +2 Vote: I do not like it

        Can you show me the case my answer gives the wrong answer?

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          4 years ago, # ^ |
            Vote: I like it -9 Vote: I do not like it

          Try 6 6 The answer is 7

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            4 years ago, # ^ |
            ← Rev. 4   Vote: I like it +22 Vote: I do not like it

            NO. The answer is 8. And like this:

            1 . 2 . 3 3
1 . 2 . . .
. . . . 4 4
5 5 . . . .
. . . 6 . 7
8 8 . 6 . 7
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              4 years ago, # ^ |
                Vote: I like it 0 Vote: I do not like it

              Oh, you are right.

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      4 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      Also thought of this during contest, but was given WA on 3, also can't find any counterexample.

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      4 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      I think the answer is (n+1)*(m+1)/6, but I didn't prove it yet.

      Apart from proving, could you please explain your intuition?

      What is a resoning behing this formula?

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        4 years ago, # ^ |
          Vote: I like it +3 Vote: I do not like it

        It is now understood. An explanation by Noam527 was helpful.

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          4 years ago, # ^ |
            Vote: I like it 0 Vote: I do not like it

          I thought it by finding that when input is 13 17, the answer is 42. :)

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            4 years ago, # ^ |
              Vote: I like it 0 Vote: I do not like it

            Are you kidding me? How could you find the right answer for this input during the contest? (13x17 seems to be too large for brute force.)

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              4 years ago, # ^ |
                Vote: I like it 0 Vote: I do not like it

              It is like this.

              A . B . C . D . E . F . G
A . B . C . D . E . F . G
. . . . . . . . . . . . .
H . I . J . K . L . M . N
H . I . J . K . L . M . N
. . . . . . . . . . . . .
O . P . Q . R . S . T . U
O . P . Q . R . S . T . U
. . . . . . . . . . . . .
a . b . c . d . e . f . g
a . b . c . d . e . f . g
. . . . . . . . . . . . .
h . i . j . k . l . m . n
h . i . j . k . l . m . n
. . . . . . . . . . . . .
o . p . q . r . s . t . u
o . p . q . r . s . t . u

              Actually not so hard..

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                4 years ago, # ^ |
                  Vote: I like it 0 Vote: I do not like it

                But how did you know that this tiling is an optimal?

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                  4 years ago, # ^ |
                    Vote: I like it 0 Vote: I do not like it

                  In fact it is another intuition. So I checked another case:

                  A A . B B . C C . D D . E
. . . . . . . . . . . . E
F F . G G . H H . I I . .
. . . . . . . . . . . . J
K K . L L . M M . N N . J
. . . . . . . . . . . . .
O O . P P . Q Q . R R . S
. . . . . . . . . . . . S
T T . U U . a a . b b . .
. . . . . . . . . . . . c
d d . e e . f f . g g . c
. . . . . . . . . . . . .
h h . i i . j j . k k . l
. . . . . . . . . . . . l
m m . n n . o o . p p . .
. . . . . . . . . . . . q
r r . s s . t t . u u . q

                  But this case gave me same answer.

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4 years ago, # |
  Vote: I like it +4 Vote: I do not like it

When you solve D problem the first time and contest becomes unrated :/

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4 years ago, # |
  Vote: I like it +1 Vote: I do not like it

The contest became unrated. :(

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4 years ago, # |
  Vote: I like it +1 Vote: I do not like it

The luckiest and unluckiest day of my life

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4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

What's the answer of this input for problem B?

4 4
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4 years ago, # |
← Rev. 2   Vote: I like it +74 Vote: I do not like it

It's 4:50 a.m. in my timezone.

And the round is unrated.

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4 years ago, # |
  Vote: I like it +28 Vote: I do not like it

How to solve B?

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4 years ago, # |
  Vote: I like it +71 Vote: I do not like it

cfblb
upload

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4 years ago, # |
← Rev. 2   Vote: I like it +13 Vote: I do not like it

BanRussiaAtIOI please stop creating problems (с) (2).

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4 years ago, # |
  Vote: I like it +12 Vote: I do not like it

Let's solve problems for fun!

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4 years ago, # |
  Vote: I like it +3 Vote: I do not like it

Scored well for the first time. Round unrated. Can't laugh anymore at my fate. LOL crying inside

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4 years ago, # |
  Vote: I like it -23 Vote: I do not like it

Round became unrated => Let's start down-voting the blog post :D

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    4 years ago, # ^ |
      Vote: I like it +66 Vote: I do not like it

    There was some problem in B, it affected a lot of people and hence unrated, doesn't mean we down vote him, it takes a lot of efforts to come up with problem ideas and write problem statements.

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      4 years ago, # ^ |
        Vote: I like it +13 Vote: I do not like it

      I know man I appreciate all the effort, I up-voted him too, but it's some kind of Codeforces tradition.

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      4 years ago, # ^ |
        Vote: I like it +5 Vote: I do not like it

      But it is also true for the problem setters to take responsibility for their mistakes.

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4 years ago, # |
  Vote: I like it +17 Vote: I do not like it

calm down...

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4 years ago, # |
  Vote: I like it +11 Vote: I do not like it

I wrote down last Div2 contest that from now on I will skip B's and work on C and D, and I managed to solve D while I didn't have even the slightest idea how to solve B (or C). Seriously guys, fix your B's and C's, C's are usually better and easier than B's because you put some stupid string/geometry tasks in B that are unnecessarily hard.

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4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

When the contest time is so convenient for you, but the round became unrated. SAD LIFE

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4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Is this round unrated now?

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    4 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    sad to say yes :(

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    4 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Can anyone give solution to problem C?

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      4 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      In Problem C, Look for every substring of length |b| in string a , Now, for every substring count the number of set bits and if the sum of set bits in a and string b are even then just add 1 to your answer.

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      4 years ago, # ^ |
        Vote: I like it +7 Vote: I do not like it

      If there's an equal amount of 1's and 0's in string b and string(a.substr(i,b)) then their difference must be even.

      1010 and 1100 is a good example, no matter how you arrange those 1's and 0's, their difference will still be even.

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4 years ago, # |
← Rev. 6   Vote: I like it +141 Vote: I do not like it

I thought that author of problem B was just unlucky. Even many (it could be over 80%, maybe?) oranges or reds are solving with incorrect solutions. So it is not to surprise that all writers and testers solved in incorrect but same way.

So, I can't gaze at the current situation which problem setter said to stop creating problems.

And I don't think that it should be heavily downvoted, as same reason as Codeforces Round #373.

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    4 years ago, # ^ |
      Vote: I like it +50 Vote: I do not like it

    For such problems, I in general expect rigorous mathematical proofs of the claims made. So, I'd like to know if the proof went wrong somewhere, or did everyone else use some greedy-like technique?

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      4 years ago, # ^ |
      ← Rev. 3   Vote: I like it +13 Vote: I do not like it

      Both are possible. It is possible that proof of problem B went wrong somewhere, judging by many orange of red coders tend to at least find a way to prove the optimality before coding.

      But it is also possible that authors "verified" their solution by brute-force check for small H and W, then got that it is same as kind of greedy-algorithm result.

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    4 years ago, # ^ |
      Vote: I like it +64 Vote: I do not like it

    I guess, just stresstesting author's solution with O($$$2^n * m$$$) will do the trick. But they are to lazy to write dp for 10 minutes or to prove something. So I think it should be heavily downvoted.

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      4 years ago, # ^ |
      ← Rev. 5   Vote: I like it +16 Vote: I do not like it

      How to calculate the optimal value with DP by $$$O(2n * m)$$$? I thought about it for five minutes and couldn't come up with it. I only came up with something like $$$O(2^{2n+α}×m)$$$ solution because we also need to think about "non-touching combination" and it will be like similar to $$$4×3$$$ tiles.

      Also, implementation is difficult. It is inevitable that tester thought that brute-force is enough for testing (if did), contrasting to heavy-implementation DP. Since the complexity of brute-force is about $$$O(2^{nm}×nm)$$$, it is no wonder they missed the minimal hacking case of $$$n=6,m=6$$$.

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        4 years ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        Ok, all right, O($$$2^{2n} * m$$$). Ok, even brute-force is enough to find 6x6 test, isn't it?

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          4 years ago, # ^ |
          ← Rev. 6   Vote: I like it +18 Vote: I do not like it

          I manage to find optimal answer of $$$n=6,m=6$$$, but it took more than $$$45$$$ seconds with pruning brute-force algorithm written in C++.

          Here is the results of all possible testcase for $$$n \leq 6, m \leq 6$$$. The results are represented as the format H, W, the optimal answer, and the number of states in pruned DFS, in one line. My program took $$$67.535$$$ seconds to compute all of them.

          Solutions

          My pruning algorithm is that "decide if there's a tile, from increasing order of pair (x,y), and prune if $$$size \geq 3$$$ 8-connected components appeared.

          I think my pruning algorithm is not perfect. But I at least can say that the situation difference between $$$(H,W)=(6,5)$$$ and $$$(6,6)$$$ is huge.

          UPD: Seems like $$$H=6, W=6$$$ was not even a minimal hacking case. I made a mistake in my calculation, and the intended answer was correct. Seems like E869120 made a more efficient brute-force (refer to this comment). But even he could not find the hacking case. It means that efficient brute-force cannot even find challenge case.

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          4 years ago, # ^ |
          ← Rev. 3   Vote: I like it 0 Vote: I do not like it

          Sorry, I made a mistake in my calculation. $$$6×6$$$ is even correct. I thought that $$$7×7÷6=7.x$$$. What is the minimal hacking case, though?

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            4 years ago, # ^ |
            ← Rev. 3   Vote: I like it +3 Vote: I do not like it

            $$$7∗7/6=8.16666666666667$$$

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              4 years ago, # ^ |
              ← Rev. 4   Vote: I like it +32 Vote: I do not like it

              Finally we managed to prove that the optimal answer will be exactly $$$⌊\frac{(H+1)(W+1)}{6}⌋$$$. The proof was rather difficult, but I liked it. It was a good example of IMO-like problem.

              I even think that the problem can re-appear, because now there exists a correct solution.

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                4 years ago, # ^ |
                  Vote: I like it +30 Vote: I do not like it

                That's not how IMO problems look like... Though I haven't proved it yet, this problem absolutely requires lots of casework and details, and that's not the spirit of IMO.

                Also, this is a nice problem, but in my opinion, it is not suitable for CP contest since most people will just guess the answer without really proved it and the difficulty of the problem is mainly its proof.

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                4 years ago, # ^ |
                  Vote: I like it 0 Vote: I do not like it

                I provide it in Chinese. https://codeforces.com/blog/entry/68048

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        4 years ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        how to solve B using dp?

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      4 years ago, # ^ |
        Vote: I like it +347 Vote: I do not like it

      You are right that this problem is not well-prepared. We could fix it. However, I absolutely do not agree that we were lazy. We spent a lot of time preparing this round. We fucked up, that is true. The main reason of this is me, because as a CF employee, I had to check everything. When we started to prepare the main contest, we did not want to hold a mirror. But in a week before the round, KAN told me that there is a need of a CF round at the end of this month. I recalled that we were organizing a contest at that period of time. Therefore, I decided to let people solve those problems too. Why do you think we do it? Why did we prepare round? Money? Shit no, if I wanted to earn money, I would prepare problem for Codechef, as many of my friends do. If we wanted to earn money from this round, we would give these problems to Codechef and earn much more. Instead of this, we decided to satisfy the need of CF users of a round. If you want to blame us because of it, go ahead, downvote the post (I am sure you did it), say that BanRussiaAtIOI fucked up.

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        4 years ago, # ^ |
          Vote: I like it +12 Vote: I do not like it

        But this round didn't even have testers

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          4 years ago, # ^ |
            Vote: I like it +22 Vote: I do not like it

          We had cross-testing. Authors of some problems tested other problems.

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            4 years ago, # ^ |
              Vote: I like it +13 Vote: I do not like it

            Well, then at least 1 tester of 5 should have written stress.

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            4 years ago, # ^ |
            ← Rev. 3   Vote: I like it 0 Vote: I do not like it

            So you didn't prepare very well.

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        4 years ago, # ^ |
          Vote: I like it +27 Vote: I do not like it

        everyone makes mistakes, it is fine, ignore the haters

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          4 years ago, # ^ |
            Vote: I like it 0 Vote: I do not like it

          Lol, that is, if you are criticized, then they are wrong?

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        4 years ago, # ^ |
          Vote: I like it +272 Vote: I do not like it

        Nobody said you're doing it for money so I don't see a point in bringing it up. Especially in a comment that you linked in the announcement blog.

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          4 years ago, # ^ |
            Vote: I like it +8 Vote: I do not like it

          So that more greedy coders start writing codechef problems instead of cf rounds :)

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        4 years ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        I would start with the basic issue: that problem was completely inappropriate as div2 B. Perhaps if it was valued correctly, it would have been tested and proofed better...

        IMHO this type of problems should be forbidden in contests at all...

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        4 years ago, # ^ |
          Vote: I like it +141 Vote: I do not like it

        Meanwhile codechef: Why the fuck are you dragging us into this

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        4 years ago, # ^ |
          Vote: I like it -13 Vote: I do not like it

        I just want to say you are really a great man, and selfless. Good luck and look forward to your next contest!

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        4 years ago, # ^ |
          Vote: I like it -76 Vote: I do not like it

        You are absolutely right! But why don't you just stop creating problems? :)

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        4 years ago, # ^ |
          Vote: I like it +5 Vote: I do not like it

        So, will you publish editorial of this round ?

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        4 years ago, # ^ |
          Vote: I like it +295 Vote: I do not like it

        So basically you are saying that it is OK to make problems with incorrect authors solution as long as it is some local contest or Codechef round?

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        4 years ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        When we start system testing ?

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        4 years ago, # ^ |
        ← Rev. 2   Vote: I like it 0 Vote: I do not like it

        BanRussiaAtIOI rest are fine but u shouldn't dragged money and codechef in between. No sense of making these points here.

        you should really improve your problem quality, first your problem c of your previous round and now this.

        && last thing codechef doesn't pay money to anyone who just come up with a random bad problem and puts it in contest without testing or juding its difficulty. .

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        4 years ago, # ^ |
          Vote: I like it +6 Vote: I do not like it

        Can you just run the system test？ I want to try other problems in the problemset.

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        4 years ago, # ^ |
          Vote: I like it +44 Vote: I do not like it

        Efforts can't win respect without taking responsibility.

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        4 years ago, # ^ |
        ← Rev. 2   Vote: I like it -10 Vote: I do not like it

        Who doesn't shoot — will never miss.)

        P.S. Ok, maybe, someone didn't understand me. I don't think anything bad about authors of the round. It was a proverb and means "Everyone who does something sometimes makes mistakes" and it NOT BAD. It is a part of process.

        Peace.

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        4 years ago, # ^ |
          Vote: I like it -10 Vote: I do not like it

        How to prove that the solution of problem B is (n+1)*(m+1)/6?

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        4 years ago, # ^ |
          Vote: I like it -10 Vote: I do not like it

        Thank you very much! The problems are really interesting

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        4 years ago, # ^ |
          Vote: I like it -6 Vote: I do not like it

        What is a lot of time by the way? 10 hours?

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        4 years ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        BanRussiaAtIOI fucked up.

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    4 years ago, # ^ |
      Vote: I like it +204 Vote: I do not like it

    So it is not to surprise that all writers and testers solved in incorrect but same way.

    I completely disagree with this statement. A setter shouldn't try to solve a problem in 5 minutes and then call it a day. It isn't the only job of testers to get AC either.

    That being said, I don't like all the "stop creating problems" comments. To all setters: just make sure you write brute force solutions, you write down proofs, and that somebody will carefully read those proofs.

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      4 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      I wholeheartedly agree. I think that with just a little bit of extra testing, the quality of CF rounds in general will increase.

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4 years ago, # |
← Rev. 2   Vote: I like it 0 Vote: I do not like it

In problem B, the result for "6 6" should be "7" and for "8 8" it should be "13". I guess it's the problem for the solution and the point for my "Wrong answer on pretest 3" * 4 and "unsuccessful hacking attempt" * 2.

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    4 years ago, # ^ |
      Vote: I like it +52 Vote: I do not like it

    For 6x6, the answer would be 8

    1 . 2 . 3 3
1 . 2 . . .
. . . . 4 4
5 5 . . . .
. . . 6 . 7
8 8 . 6 . 7
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      4 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      Well, my fault...So can this problem be solved in O(1) time?

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        4 years ago, # ^ |
          Vote: I like it +11 Vote: I do not like it

        It might be

        ( n + 1 ) * ( m + 1 ) / 6

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          4 years ago, # ^ |
            Vote: I like it +113 Vote: I do not like it
          Yes
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            4 years ago, # ^ |
              Vote: I like it +3 Vote: I do not like it

            Looks right, but I could not prove the very first claim myself and then gave up. How to prove that if there is a correct placement of the 1x2 and 2x1 pieces, then the extended 2x3 and 3x2 pieces do not overlap?

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              4 years ago, # ^ |
                Vote: I like it +2 Vote: I do not like it

              Consider most high right point these usual pieces touch. If it belongs to both pieces, they overlap. Now it belongs to one "hightest" piece. They overlap iff extended highest overlaps other one's highest rightmost point. It cannot overlap other one's extended side (the one we added, imaginary)

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                4 years ago, # ^ |
                  Vote: I like it 0 Vote: I do not like it

                Thanks! Still can't say I completely understood it but I got the idea.

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                  4 years ago, # ^ |
                    Vote: I like it 0 Vote: I do not like it

                  if extended higher upper piece touches other usual piece, then both extended pieces collide.

                  If it does not touch other usual piece, it does not touch its extension: (Same as if extended touch, usual pieces touch) If extension covers usual piece, then both usual pieces touch trivial. If both extended areas touch, then: a) Going higher on piece 2 collision (that is lower or right-er) from that point, we get to usual piece from piece 2, that piece also collides with extended 1, trivial case above. It cant go higher because piece 2 is lower. Also, all higher pieces collide and if there are no usual pieces 2 on that path, we can still use that fact. Let's take highest such point b) We get some other extended collision piece. By same argument, we can go left and find usual piece from 2 that collides with extended 1. This time, we know that going left once will get us to usual piece 2 (there is only 1 point with the property for both types of extended pieces). This time, this piece may not even touch extended 1, but be in lower left corner of it. So it touches usual 1 by our rules anyways (this one is extra case, others are trivial).

                  Extended = piece with border. Extension = border (down and right) usual piece = original one, without extension

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            4 years ago, # ^ |
            ← Rev. 3   Vote: I like it 0 Vote: I do not like it

            Thank you for the detailed explanation

            This is the 7x7 case: (in case anyone was wondering)

            7x7
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              4 years ago, # ^ |
                Vote: I like it 0 Vote: I do not like it

              What does that state mean? I can't find the purpose of that state because the answer of input 7 7 is 10; like this:

              A . B . C . D
A . B . C . D
. . . . . . .
E . F . G . H
E . F . G . H
. . . . . . .
I I . J J . .

              and other solution also exists.

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                4 years ago, # ^ |
                  Vote: I like it 0 Vote: I do not like it

                In fact we say "7 7" means H+1=7 and W+1=7. So you should use "6 6" as input.

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                4 years ago, # ^ |
                  Vote: I like it 0 Vote: I do not like it

                It was in response to Noam527's comment. I think saying n=7 m=7 would be better. It's for a 6x6 grid

                From that convert 2x3 blocks into 1x2 blocks and 3x2 to 2x1

                1 * 2 * 3 3
1 * 2 * * *
* * * * 4 4
8 8 * * * *
* * * 6 * 5
7 7 * 6 * 5
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        4 years ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        This has to be solved in O(1) time.

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    4 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    me the same. my solution is (m+1)(n+1)/6 but "Wrong answer on pretest 3". I dont know why :((

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      4 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      Because the data of the problem was wrong

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4 years ago, # |
  Vote: I like it +15 Vote: I do not like it

Delayed 15 min and turned out to be unrated. What a good round!!!

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    4 years ago, # ^ |
      Vote: I like it +14 Vote: I do not like it

    Something was fishy about this round from the start.

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      4 years ago, # ^ |
        Vote: I like it +30 Vote: I do not like it

      From the announcement. You could see BanRussiaAtIOI as author

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        4 years ago, # ^ |
          Vote: I like it +3 Vote: I do not like it

        It's ok to complain about the contest, and downvote the blog, because he made a big mistake.

        But I don't see any reason to make personal attacks. If you can't find a way to give constructive feedback, just shut up.

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4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

A problem in EGMO contest 2019 is just a special case of Problem B today (they just ask to find the maximum with table of size 2n x 2n):

https://artofproblemsolving.com/community/c6t520112f6h1818715_yet_another_domino_problem

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    4 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Nope. Those dominoes have to be 2 blocks away not 1. This problem is more similar to Croatian nationals 2019 where I would have won gold if I didn't misread a problem. (The problem is #3 on page 3, use google translate)

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4 years ago, # |
← Rev. 3   Vote: I like it +17 Vote: I do not like it

What a shitty contest.Problem A kindergarten problem.Problem B is shit. Problem D is easier than B. Don't know who is testing these rounds.

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    4 years ago, # ^ |
      Vote: I like it -22 Vote: I do not like it

    Agree problem B was shit. And A was very tough :(

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    4 years ago, # ^ |
      Vote: I like it +71 Vote: I do not like it

    I agree with you, but please respect the authors. Preparing a round is difficult.

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    4 years ago, # ^ |
    ← Rev. 2   Vote: I like it +8 Vote: I do not like it

    I also agree that B in this game is very bad.

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4 years ago, # |
  Vote: I like it +32 Vote: I do not like it

What the f**k?

I had a great score in this round and I can be a candidate master then it is unrated?

I thank the authers very mucn and I know preparing a round is difficult. But I still think this is a little sad.

Hope everyone can have a great score next time ans hope myself can be a candidate master. :(

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    4 years ago, # ^ |
    ← Rev. 2   Vote: I like it 0 Vote: I do not like it

    Today, somehow I managed to do the problem D and the contest got unrated. karma is a bitch!!

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    4 years ago, # ^ |
    ← Rev. 2   Vote: I like it +6 Vote: I do not like it

    Come on. Keep training. Hope you can get a gratifying score in the next round. (And hope there isn't any grammar mistake in those sentences)

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4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

A perfect contest becomes bad because of problem. I feel upset.

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    4 years ago, # ^ |
      Vote: I like it +9 Vote: I do not like it

    How was the contest perfect in the first place?

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4 years ago, # |
  Vote: I like it +34 Vote: I do not like it

I am very very disappointed in codeforces. I woke up at 3:30 AM just to do this contest. I was actually pretty happy when I saw the problems because I knew how to solve them and I was predicted to get quite a bit of positive delta. But all of a sudden announcing that the round is unrated because the problem writers for some reason can't make good problems is quite discouraging. Imagine how many times I would have wanted round to be unrated in order to not lose rating. Maybe this should be a wake up call for all those creating contest: From weak pretests, to weak solutions, to weak language. All this can be avoided by testing thoroughly and thoughtfully.

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4 years ago, # |
  Vote: I like it +4 Vote: I do not like it

Weird difficulty distribution.
For me it is just like A < C < D < F < both B and E
What thE HeLL???

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    4 years ago, # ^ |
      Vote: I like it +1 Vote: I do not like it

    Dude, I had to use SCAN to pass F, wtff ????

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      4 years ago, # ^ |
        Vote: I like it +7 Vote: I do not like it

      My dumbest greedy algo' passed the pretests while I can prove it is neither right nor wrong lmaooooo

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4 years ago, # |
← Rev. 2   Vote: I like it +19 Vote: I do not like it

finally passed pretest of F have almost +200 on predictor, excited to become candidate master. And the next second it became unrated.....so upset.

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4 years ago, # |
← Rev. 2   Vote: I like it +17 Vote: I do not like it

real shit contest ! hard B(even writer and many unofficial participant don't have solution). fucking implementation D and E with Case handling !

please when you are writing problems be in participants shoes and don't write shitty problems !!!

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4 years ago, # |
  Vote: I like it +45 Vote: I do not like it

How does it feel when 6 authors + testers + coordinators can't properly solve 7th grade math?

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    4 years ago, # ^ |
      Vote: I like it +37 Vote: I do not like it

    Moreover, there are 2 gold IOI medalists and 3 IMO medalists among them

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      4 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      Why is it so hard to make good problems? I am genuinely curious.

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4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

By the way, it was because of the problem B that I moved to D and could solve it. So, thanks anyway.

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4 years ago, # |
← Rev. 2   Vote: I like it +215 Vote: I do not like it

UNRATED

That's the result when you forgot to thank MikeMirzayanov for codeforces and polygon platform. :)

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    4 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    it is the K A R M A T R I G G E R E D
    (nope

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4 years ago, # |
  Vote: I like it 0 Vote: I do not like it

The problem setter of B should stop making questions. At last it became unrated. This was my debut round for becoming specialist XD. Someone wants to see me in green.

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4 years ago, # |
← Rev. 2   Vote: I like it +4 Vote: I do not like it

OK,I can solve these problems for fun.But in fact it waste my time.But I won't decrease my rating.

It's a lucky and unlucky day.

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4 years ago, # |
  Vote: I like it +3 Vote: I do not like it

Can someone give me counterexample to (a+1)*(b+1)/6 in B?

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    4 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    As someone already pointed out, 6x6 can have upto 8 dominoes placed.

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      4 years ago, # ^ |
        Vote: I like it +5 Vote: I do not like it

      7*7/6=8

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        4 years ago, # ^ |
        ← Rev. 2   Vote: I like it 0 Vote: I do not like it

        Ah, my bad. Consider 5x5 then — we expect 5*5/6 = 25/6 = 4 as the answer, but you can get 5.

        Edit: Oops, I'll just let someone else answer this before I make more blunders now :(

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          4 years ago, # ^ |
            Vote: I like it 0 Vote: I do not like it

          In 5x5 you can even have 6, and 6*6/6=6 .

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    4 years ago, # ^ |
    ← Rev. 2   Vote: I like it 0 Vote: I do not like it

    6x5? I think answer is 6.

    (n+1)*(m+1)/2 gives 7.

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      4 years ago, # ^ |
      ← Rev. 2   Vote: I like it 0 Vote: I do not like it

      **.*.*

      ...*.*

      **....

      ...*.*

      **.*.*

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      4 years ago, # ^ |
        Vote: I like it +1 Vote: I do not like it

      5x6 is 7. 

      11.2.3  
...2.3  
44....  
...5.6  
77.5.6

      this is one possible way to reach that

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    4 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    I think the input 4 10 does not work. The answer is supposed to be 8 but the formula gives (5 * 11) / 6 = 55 / 6 = 9. Here are some possible arrangements of 8 tiles in a 4 * 10 matrix:

    1 1 . 2 2 . 3 3 . .
. . . . . . . . . .
4 . 5 . 6 . 7 . 8 .
4 . 5 . 6 . 7 . 8 .


    1 1 . 4 . 5 5 . 8 .
. . . 4 . . . . 8 .
2 . . . . 6 . . . .
2 . 3 3 . 6 . 7 7 .

    I cannot find any arrangement for 9 tiles.

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      4 years ago, # ^ |
        Vote: I like it +13 Vote: I do not like it 
      1 . 2 . 3 . 4 . 8 8
1 . 2 . 3 . 4 . . .
. . . . . . . . . 9
5 5 . 6 6 . 7 7 . 9
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4 years ago, # |
  Vote: I like it +1 Vote: I do not like it

The problem statement of B was very unclear. Did C and D and later found that the round was unrated. Many contestants like me are very excited whenever we see a new contest on the contest page and are equally disappointed when the round becomes unrated due to such reasons.

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4 years ago, # |
  Vote: I like it +11 Vote: I do not like it

waste time for this contest, 1 min to solve A and 1 hour can't solve shit problem B. And when it remove, i spend 5 min to solve D -_-

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    4 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    That's why I always read B, C, D after solving A. You never know, maybe D is easier than B XD

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4 years ago, # |
  Vote: I like it +6 Vote: I do not like it

Why "this round be the UNRATED"? Other problems are perfect. :(

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    4 years ago, # ^ |
    ← Rev. 2   Vote: I like it +17 Vote: I do not like it

    Because many people waste too much time in B,I think

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    4 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    I stuck with B because my solution is right

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    4 years ago, # ^ |
      Vote: I like it +5 Vote: I do not like it

    Last time, one similar contest also became unrated just because the problem A had a small mistake on a special situation. And the mistake was discovered and fixed in less than 15 minutes. But the round still became unrated.

    So any mistake which may influence the contestants' score and rating changes is not allowed to be found in a rated Codeforces round. That's the rigorousness of Codeforces.

    By the way, if you exactly participate a round like this and you lose the high rating delta, looking forward to next round and try to keep a good mood.

    Hope everybody get a good score in the next round.(And hope there isn't any grammar mistake in those sentences)

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4 years ago, # |
  Vote: I like it -40 Vote: I do not like it

Blacklist all problem setters.. :'/

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4 years ago, # |
  Vote: I like it +11 Vote: I do not like it

My mood right now after knows the contest is unrated

mood

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4 years ago, # |
  Vote: I like it +6 Vote: I do not like it

Who is here after the round being unrated and before the end of the contest?

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4 years ago, # |
  Vote: I like it +41 Vote: I do not like it

The problem maker is cxk

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