Educational Codeforces Round 87 [Rated for Div. 2]
By awoo, history, 9 months ago, translation, In English

Hello Codeforces!

On May/17/2020 12:20 (Moscow time) Educational Codeforces Round 87 (Rated for Div. 2) will start.

Please notice the unusual time.

Series of Educational Rounds continue being held as Harbour.Space University initiative! You can read the details about the cooperation between Harbour.Space University and Codeforces in the blog post.

This round will be rated for the participants with rating lower than 2100. It will be held on extended ICPC rules. The penalty for each incorrect submission until the submission with a full solution is 10 minutes. After the end of the contest you will have 12 hours to hack any solution you want. You will have access to copy any solution and test it locally.

You will be given 6 or 7 problems and 2 hours to solve them.

The problems were invented and prepared by Roman Roms Glazov, Adilbek adedalic Dalabaev, Vladimir vovuh Petrov, Ivan BledDest Androsov, Maksim Neon Mescheryakov and me. Also huge thanks to Mike MikeMirzayanov Mirzayanov for great systems Polygon and Codeforces.

Also thanks to Neal neal Wu for testing.

Good luck to all the participants!

Our friends at Harbour.Space also have a message for you:

Codeforces and Harbour.Space

Hi Codeforces!

We would like to invite you to a very special webinar called Digital Lockdown: AI against COVID-19, by Sergey Gordeychik, director of our Cyber Security programme.

Sergey is CIO of Inception Institute of Artificial Intelligence, and former CTO at Kaspersky.

During this webinar, Sergey will share his expertise and insights on how AI is being used both positively and negatively, during the COVID-19 global pandemic. Tune in for some practical examples of how companies are using AI to innovate and disrupt during a time of crisis, exploring topics like Medical Imaging for CT analysis, diagnosis and mass surveillance.

Join us on Thursday, May 28th at 12h (BCN) to gain knowledge and deepen your understanding about how we can use AI to solve both operational and societal problems.

By participating in this 1hour webinar you will get a certificate of participation, a special digital gift from Sergey, and have the chance to win a FREE 3-week module at Harbour.Space University, depending on the availability and prerequisites of the course.

Reserve your spot now!

Congratulations to the winners:

Rank Competitor Problems Solved Penalty
1 square1001 8 294
2 Anadi 8 305
3 tfg 8 681
4 kefaa2 7 192
5 xay_naive 7 248

Congratulations to the best hackers:

Rank Competitor Hack Count
1 qwscaln 29:-2
2 Ankit 5
3 lvao-x 3:-1
4 the_redback 3:-1
5 WICK_ED 2:-1
142 successful hacks and 828 unsuccessful hacks were made in total!

And finally people who were the first to solve each problem:

Problem Competitor Penalty
A fedoseev.timofey 0:02
B Ashishgup 0:03
C1 hitman623 0:04
C2 square1001 0:15
D Not-Afraid 0:10
E autumn_eel 0:17
F squarepants 0:47
G riantkb 0:37

UPD: Editorial is out

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

Comments Comments (746)
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9 months ago, # |
← Rev. 2   Vote: I like it -80 Vote: I do not like it

Good Luck ..

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9 months ago, # |
  Vote: I like it +46 Vote: I do not like it

Wow this is really a good time in the Philippines! We can finally experience to join a contest at 5PM

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Again! Unusual time.

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9 months ago, # |
  Vote: I like it +18 Vote: I do not like it

Codeforces is entertaining quite by regularly holding contests in Lockdown. I'm enjoying. What about you guys?

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9 months ago, # |
  Vote: I like it -11 Vote: I do not like it

Good Luck & Have Fun!

Hope the round will go smoothly like last round!

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9 months ago, # |
  Vote: I like it +135 Vote: I do not like it

awoo , MikeMirzayanov with all due respect , can you please reschedule the round to next day or during usual time since kickstart will start 5 minutes before this round will end . We can't code for 5 hours straight .

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    9 months ago, # ^ |
      Vote: I like it +54 Vote: I do not like it

    Did you make this account just to make comments?? I wonder what could be the psychology behind this o_O

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      9 months ago, # ^ |
        Vote: I like it +28 Vote: I do not like it

      Making alt accs just to make comments help as it helps you to be the jerk you are without ruining your real reputation. Though this comment isn't exactly being a jerk.

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        9 months ago, # ^ |
          Vote: I like it -27 Vote: I do not like it

        You have my upvote.

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        9 months ago, # ^ |
          Vote: I like it +13 Vote: I do not like it

        meeooow can you plz try to achieve 0 ranting I really really wanna know if it is possible or not

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

          Yes it's possible, even negative rating is possible. The lowest rating right now is -41. Check the last page of ratings table

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          9 months ago, # ^ |
            Vote: I like it +10 Vote: I do not like it

          yep. trying my best to reach -ve asap. Though there are quite a few of them who've reached this milestone before. :)
          It's "rating", and not "ranting" btw.

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          9 months ago, # ^ |
            Vote: I like it 0 Vote: I do not like it

          Ratings can also be negative.

          I've seen a few negative rated profiles.

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        9 months ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        @meeooow pretty cool rating graph btw

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    9 months ago, # ^ |
      Vote: I like it +11 Vote: I do not like it

    I like your contribution :P

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    9 months ago, # ^ |
      Vote: I like it -13 Vote: I do not like it

    Did you make a new Codeforces account to say this?

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    9 months ago, # ^ |
      Vote: I like it +12 Vote: I do not like it

    I agree. Yesterday's Round 643 was stressful too which affected the Codejam after it. I would hate to skip a codeforces round especially made by such good writers.

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

    :( .contest delayed by 10+5 minutes.Now 20 minutes of overlap .

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      9 months ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      5 more minutes. But why?

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      9 months ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      I'm pretty sure you can just close the tab and go to Kickstart.

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        9 months ago, # ^ |
          Vote: I like it +1 Vote: I do not like it

        Well, if you perform poorly in today's round that will affect your kickstart participation so it's not just about time.

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    9 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    And now the constest is delayed by 15 minutes more. Not a pro coder that I can solve all here and then go for kickstart as well.

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    9 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Seems like they deliberately wanted the contests to overlap. The 15 minutes postpone just strengthens this hypothesis.

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9 months ago, # |
  Vote: I like it +24 Vote: I do not like it

Whatever time it is the participants always show up in large numbers. I'm proud of this community :)

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9 months ago, # |
  Vote: I like it +62 Vote: I do not like it

Hey, this will clash with Kickstart round C for the last 5 minutes of contest, could you please schedule it something like 10 minutes earlier?

Thanks

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9 months ago, # |
  Vote: I like it +7 Vote: I do not like it

Back to back contests.. That's why I love codeforces.

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

It's great that there are so many contests but why this unusual timings ??? It's just irritating.....

Back to back unusual time contests !!! Oohhh no...

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Does the hacking phase got any points? Or will it affect the rating?

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    9 months ago, # ^ |
      Vote: I like it -15 Vote: I do not like it

    Go google it kid.

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    9 months ago, # ^ |
      Vote: I like it +10 Vote: I do not like it

    No. Educational and div3,4 hacking phase hacking has no points.

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9 months ago, # |
  Vote: I like it +6 Vote: I do not like it

I have rarely seen unusual time in educational rounds. What is the reason for unusual time?

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    9 months ago, # ^ |
    ← Rev. 2   Vote: I like it +7 Vote: I do not like it

    I am not very sure but as far as I know, the contest time depends on the availability of the setters/testers during the contest. So the reason might be the mutual time decided by the setters/testers as their available time. Someone correct me if I am wrong.

    EDIT: It seems one of the setters has already notified the reason. This contest is tied to a local contest in Saratov.

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    9 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    IDK

    Maybe because of some other coding events(or contests)

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9 months ago, # |
  Vote: I like it +33 Vote: I do not like it

I think it's clashing with Google Kickstart round C. Though just 5 minutes but still it matters

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    9 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    There is no break between the two contests that is only problem, usual time would't have created this problem. At last it is up to problem setters, they have their reasons.

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      9 months ago, # ^ |
        Vote: I like it -10 Vote: I do not like it

      Can someone help me with the problems of kickstart round C. Where can I find the editorial?

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9 months ago, # |
  Vote: I like it +1 Vote: I do not like it

Will it be easier than general codeforces round of div 2 ? (I have fewer experience about educational round )

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    9 months ago, # ^ |
      Vote: I like it +12 Vote: I do not like it

    I feel educational rounds slightly more difficult than regular div 2.But be sure to learn newer things.

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    9 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    maybe harder cause its problems is from GCJ

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    9 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    from my experiences the difficulty is pretty much similar. but the open hack phase introduces some new challenges for us

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9 months ago, # |
  Vote: I like it +3 Vote: I do not like it

This contest has a clash of 5 minutes with Kickstart Round C. It would be better if this contest gets preponed by 5 minutes or so.

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9 months ago, # |
  Vote: I like it +116 Vote: I do not like it

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9 months ago, # |
  Vote: I like it -41 Vote: I do not like it

I kindly ask newbies and pupils not to participate in this round, you are causing big queues. you are wasting time. Stop programing!!!

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9 months ago, # |
  Vote: I like it -16 Vote: I do not like it

dsn

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Respected awoo BledDest MikeMirzayanov vovuh sir,

I want to bring it to your notice that today there were back to back codeforces contest and google codejam for many of us and tommorow also we are having google kickstart and educational round as well, but the problem tommorow is that both of the contest clashes. Today there was a gap of 25 minutes bectween the two but tommorow its -5 min i.e. it clashes with each other. Moreover there isn't any upcoming schedule (except kotlin heros) so it can easily be shifted on some other day. If you still think that contest should be organised tommorow then please give a gap of at least 30 minutes so that everyone could refresh and take part in second one with fresh and cool mind and sorry I am bit shy. Please ignore and forgive me if I am pointless.

Thank you

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

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9 months ago, # |
  Vote: I like it +119 Vote: I do not like it

Unfortunately, we cannot shift the round time because it is tied to a local contest in Saratov.

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    9 months ago, # ^ |
      Vote: I like it -36 Vote: I do not like it

    RIP our concentration and brain. Thanks for letting us know

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9 months ago, # |
  Vote: I like it +146 Vote: I do not like it

Codeforces evolution ->

2017 — Spamming "Is it rated?"

2020 — Spamming unfunny memes

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    9 months ago, # ^ |
      Vote: I like it +19 Vote: I do not like it

    I feel it's my fault :P

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      9 months ago, # ^ |
        Vote: I like it +3 Vote: I do not like it

      No bro you are the legendary grandmemer of codeforces ,soon you will be among the top contributors

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        9 months ago, # ^ |
          Vote: I like it +13 Vote: I do not like it

        Thx <3 but Nah I won't reach top contributors the higher your contribution is the harder it gets to gain more and I'm running out of memes

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          9 months ago, # ^ |
            Vote: I like it 0 Vote: I do not like it

          Ngl, I look out for that one single meme of yours in every contest announcement. They really make my day. :D

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9 months ago, # |
  Vote: I like it +13 Vote: I do not like it

Kickstart -> Educational -> Atcoder. This is just usual Sunday !!!. what's the hurry ?

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

so should we just abandon last 5 minutes of this contest for kickstart ?

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

I love Educational Round <3

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9 months ago, # |
  Vote: I like it -42 Vote: I do not like it

Is it rated?

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    9 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    It will be rated for the participants with rating lower than 2100

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

How do I unregister from this round?

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Nice contest time :)

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Finally, I didn't do codeforces in the early morning. This unusual time is too nice for me

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9 months ago, # |
  Vote: I like it +2 Vote: I do not like it

It's a good time for Chinese people, but it may not be good for other places :)

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Should I skip this round for kickstart or should I give both. Any suggestions?

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

Live epic upsolving after the round:

https://youtu.be/SdwVz4qzhkY

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

Hey, how come no more non-Kotlin Heroes contests scheduled after this Educational round? (Like dang, many hours past the contest...)

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9 months ago, # |
  Vote: I like it +18 Vote: I do not like it

Just meme :-) Pics-Art-05-17-01-11-34

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9 months ago, # |
  Vote: I like it +5 Vote: I do not like it

I think codeforces community is trying to know which time is the best time for holding contest so we don't encounter any problems. I think it is the strategy to find the most suitable time.

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9 months ago, # |
← Rev. 2   Vote: I like it -9 Vote: I do not like it

Is there any specific reason for UNUSUAL TIME or its just fun? :)

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9 months ago, # |
  Vote: I like it +15 Vote: I do not like it

Hope this contest will have some graph and DS problems.

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9 months ago, # |
  Vote: I like it +3 Vote: I do not like it

Great time!! Thanks for arranging contests so frequently . Thanks in advance for the webinar.

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9 months ago, # |
  Vote: I like it -6 Vote: I do not like it

If possible, set the time in such way that it does not clashes with another contest.

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

A really good time for Chinese! We can enjoy the contests this weekend without staying up late.

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9 months ago, # |
  Vote: I like it -6 Vote: I do not like it

It's the first time I am sitting for contests back to back. Hope the results will be good.

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9 months ago, # |
  Vote: I like it -7 Vote: I do not like it

Less than 20k participants after a long time . Results of Kick start clash may be . Looking forward for an another interesting problemset .

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9 months ago, # |
  Vote: I like it -67 Vote: I do not like it

I thing, it's good time for Bangladeshi Muslims. Because, in evening, we have evening prayer and Ifter. And in night, we have night prayer with Salatut Tarawih. But, this time,most of us are free. By the way, thank you Codeforces.

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9 months ago, # |
  Vote: I like it -15 Vote: I do not like it

Contest Extended!

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9 months ago, # |
  Vote: I like it +194 Vote: I do not like it

The contest got delayed 10 minutes and now the overlap with Google Kickstart is 15 minutes

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9 months ago, # |
  Vote: I like it +3 Vote: I do not like it

Delayed for 10 minutes.

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9 months ago, # |
  Vote: I like it +7 Vote: I do not like it

delayed :(

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

Delayed by 10 minutes!!!

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

The voltage increases... another ten minutes.

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9 months ago, # |
  Vote: I like it +8 Vote: I do not like it

what the hell is going on ??

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9 months ago, # |
  Vote: I like it +5 Vote: I do not like it

Damn it, it's getting even worse now we have to abandon last 15 minutes of this contest

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    9 months ago, # ^ |
      Vote: I like it +19 Vote: I do not like it

    Codeforces should have preponed this round or kept at 8:05, owing to know Kickstart's schedule. :)

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Delayed 10 mins :)

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9 months ago, # |
← Rev. 2   Vote: I like it +9 Vote: I do not like it

A penalty of 10 mins to everyone!! HAHA Another 5 mins! Congratulations

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

Delayed by 10 mins : hope server is fine

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9 months ago, # |
  Vote: I like it +5 Vote: I do not like it

Now a 15 minute overlap with kickstart,rather than 5

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9 months ago, # |
  Vote: I like it +5 Vote: I do not like it

I don't think it's very sensible to further delay the contest with an overlap of 15 minutes with Kick-start (which is a fairly popular competition and has almost 10k+ participants this year..)

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9 months ago, # |
← Rev. 2   Vote: I like it +68 Vote: I do not like it

Everyone said make it 5 min earlier. Boom you delay it by 10 min. People start worrying for Kickstart. Meanwhile Codeforces : "Here comes another 5 min, you guys just probably spamming"

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9 months ago, # |
  Vote: I like it +7 Vote: I do not like it

let's hope this delay is not a sign of long queues during the contest

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9 months ago, # |
  Vote: I like it +7 Vote: I do not like it

Contest should be postponed.It directly overlaps with Google Kickstart 2020.

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9 months ago, # |
  Vote: I like it -24 Vote: I do not like it

15 minutes of kickstart already lost Can't you see awoo

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9 months ago, # |
  Vote: I like it +1 Vote: I do not like it

Google Kick Start! :(

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9 months ago, # |
  Vote: I like it +123 Vote: I do not like it

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9 months ago, # |
  Vote: I like it +20 Vote: I do not like it

Is this a measure by the problem-setters to decrease their competitors in Kickstart? >.<

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    9 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    In that case, people will do Kick Start instead of this contest.

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      9 months ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      I don't think so, Kickstart has it's prestige but to people who want rated contests, they might skip Kickstart(or join late).

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9 months ago, # |
  Vote: I like it +21 Vote: I do not like it

For the First time, a 100 minute contest! XD

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9 months ago, # |
  Vote: I like it -6 Vote: I do not like it

Amazing.

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9 months ago, # |
  Vote: I like it -6 Vote: I do not like it

Another 5 mins....

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

again

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

yo come on xd

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9 months ago, # |
  Vote: I like it +39 Vote: I do not like it

okay now i am probably gonna take part in atcoder instead of kickstart (:

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    9 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    As per my suggestion prefer kickstart as atcoder contest take place every sunday but not so frequent for kickstart

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    9 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Same ^_^

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

Another 5 mins :(

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9 months ago, # |
  Vote: I like it +5 Vote: I do not like it

This is frustrating

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9 months ago, # |
  Vote: I like it +2 Vote: I do not like it

Now , i m not participating

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9 months ago, # |
  Vote: I like it +9 Vote: I do not like it

Man, You guys are killing me. Delayed further by 5 Mins. Its a 20 min overlap now.

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9 months ago, # |
  Vote: I like it +11 Vote: I do not like it

Please make sure not to start to early ;)

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9 months ago, # |
  Vote: I like it +8 Vote: I do not like it

I am truly amazed by the arrangements.

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

WTF is happening. Again 5 min delay.

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Why delayed again?

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9 months ago, # |
  Vote: I like it +5 Vote: I do not like it

JUST TELL ME THE FINAL TIME OF STARTING CONTEST. GETTING PRETTY ANNOYED BY CHANGING TIME.

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9 months ago, # |
  Vote: I like it +13 Vote: I do not like it

I want my 20 minutes back

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

I was already ready to go in, as here again the transfer for 5 minutes )

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    9 months ago, # ^ |
      Vote: I like it +14 Vote: I do not like it

    By using GP (infinite) 10+5+2.5....=20 so contest will have 20 minutes delay

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9 months ago, # |
  Vote: I like it +7 Vote: I do not like it

just forget about Kick Start

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

What's up with the delays?

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Seems its a trap now :)

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9 months ago, # |
  Vote: I like it +12 Vote: I do not like it

Why are they playing with our feelings?

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9 months ago, # |
  Vote: I like it +5 Vote: I do not like it

my patience is giving up .

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9 months ago, # |
  Vote: I like it +6 Vote: I do not like it

What the hell is that,2 delays?

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9 months ago, # |
  Vote: I like it +3 Vote: I do not like it

The contest is delayed by 15 minutes, so now we have 20 minutes intersection with Kickstart. Tough spot to be in.

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9 months ago, # |
  Vote: I like it +1 Vote: I do not like it

Here we go again!

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9 months ago, # |
  Vote: I like it +5 Vote: I do not like it

10 minute delay had a bonus!

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9 months ago, # |
  Vote: I like it +22 Vote: I do not like it

Again Delayed! It hurts more than a breakup.

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

WTF dude! Please check your server and conduct this round after kick start....

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9 months ago, # |
  Vote: I like it +25 Vote: I do not like it

While(onClick("Start Contest") { startTime+=5; }

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

5 more minutes ?

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

1 is okay but 2 is a bit frustrating

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Server in trouble?

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9 months ago, # |
  Vote: I like it +8 Vote: I do not like it

delay the round without notice is really uncomfortable...

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Are you guys planning to delay it over and over so that it starts exactly at the same time as Kickstart?

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9 months ago, # |
  Vote: I like it +6 Vote: I do not like it

wtf???????

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

why is starting time getting postponed every 5 minutes?

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

I wonder what had happened to this round.

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Now 20 min penalty woaah why cf ??

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

What should be the priority kickstart or codeforces ??

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Why are you extending the time again and again. I have been waiting for 30 minutes for this contest t begin.

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

why the contest is getting late???

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    9 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Could be technical issues or unclear problem statements.

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9 months ago, # |
  Vote: I like it +8 Vote: I do not like it

Now we'll get to see real CF fans

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9 months ago, # |
  Vote: I like it +3 Vote: I do not like it

Maybe they are waiting for 20K registrants.

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Another 5 min, Hope clear statements !

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

delayforces /qiang

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Now delay the contest to some other day THIS IS NOT DONE!!

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9 months ago, # |
  Vote: I like it +9 Vote: I do not like it

How to solve A?

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    9 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Just do binary search for the time

    first 10 min then 5 then 2 then 1

    glhf

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

why delay again !???

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9 months ago, # |
  Vote: I like it +5 Vote: I do not like it

When I say myself,**"Now we go",**then Suddenly I see 10 min left,and after 10 min ,again I boosted myself ,and again I see 5 min to go

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

I am getting a meme feeling

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9 months ago, # |
  Vote: I like it +61 Vote: I do not like it

Will they keep delaying it until unusual time becomes usual ?

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

I already ready for program but codeforce said please hold patient .

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Will educational rounds have points for hacking??

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9 months ago, # |
  Vote: I like it +12 Vote: I do not like it

Testing our patience? xD

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Kickstart guys going to suffer

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9 months ago, # |
  Vote: I like it +7 Vote: I do not like it

Guys, don't mess with cf. They will add 5 minutes delay for each meme xD

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

a contest with 100 min xD (until kickstart begins)

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Why has the contest been postponed two times?

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9 months ago, # |
  Vote: I like it +3 Vote: I do not like it

I am wondering how are you holding an onsite contest during the COVID-19 epidemic?

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9 months ago, # |
  Vote: I like it +14 Vote: I do not like it

UnusualtimeForces × DelayForces √

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9 months ago, # |
  Vote: I like it +8 Vote: I do not like it

delayforces :(

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9 months ago, # |
  Vote: I like it +3 Vote: I do not like it

Yet another 5 minutes extended.

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

They should either reduce the length of the contest for everyone or postpone the contest altogether (IMO).

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Putinforces?

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Is it rated?

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

10min+5min delay.

Hope all goes well.

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9 months ago, # |
  Vote: I like it -18 Vote: I do not like it

No offense!! Participate in kickstart. Nothing is left with educational rounds :/

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

for(;time<=4:30;start_time+=5)time++;

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

If they add 5 min more, I'll wait 5 min more

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

SPEEDFORCES!!!

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Time limit exceed in test case 3 in A problem :((

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

how to solve D? my q*lgn*lgn using binary indexed trees(bit) ofcourse times out :(

UPD — my same code with cin cout passes?!

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    9 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    you don't need binary search, traverse the BIT to find the answer ~810ms

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      9 months ago, # ^ |
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      And I kept getting Memory Limit exceeded for some reason!!

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    9 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Use binary jumps

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    9 months ago, # ^ |
      Vote: I like it +23 Vote: I do not like it

    Either fast descent on BIT or segment tree (in $$$\log n$$$), or the following:

    Suppose we are trying to find the minimum element in the resulting multiset. To do this, let's implement a function that counts the number of elements not exceeding $$$x$$$ in the resulting multiset in $$$O(n + q)$$$ as follows: if we distinguish only between elements greater than $$$x$$$ and not greater than $$$x$$$, we can maintain the multiset as two simple counters. To find $$$k$$$-th statistics, simply check that the number of elements $$$\le x$$$ is not less than $$$k$$$; if it is so, $$$k$$$-th statistics is not greater than $$$x$$$.

    That way, we can count the number of elements $$$\le x$$$ in $$$O(n + q)$$$, and to find the minimum element in the resulting multiset, we may binary search the first $$$x$$$ such that the number of elements $$$\le x$$$ is non-zero.

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    9 months ago, # ^ |
      Vote: I like it +6 Vote: I do not like it

    My q*log^2(n) luckily passed, please hack it https://codeforces.com/contest/1354/challenge/80496838

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    9 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    I used binary search on Binary Indexed trees(BIT) and it passed. Here's my submission in Java 80546129

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9 months ago, # |
  Vote: I like it +14 Vote: I do not like it

How to solve C?

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    9 months ago, # ^ |
    ← Rev. 7   Vote: I like it 0 Vote: I do not like it

    I think $$$\cot(\pi/n)$$$ works for C1.

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    9 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    For C1,i solved it by finding the inradius of the polygon multiplied by 2.

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

    1.0/tan(pi/n)

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    9 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    C1: 2 times the apothem of the polygon, meaning 

    tan(pi*(2*n - 2)/(4*n))

    C2: I have literally no ideea.

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

    C1: 1/tan(π/(2n))

    C2: cos(π/(4n))/sin(π/(2n))

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      9 months ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      can you explain problem C2

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

      is binary search intended solution for C1,C2?

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      9 months ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      Can you explain the solution of C2 ?

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      9 months ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      Hey can u explain one thing tan(angle) = tan(angle*(pi/180))

      then in case of tan(pi/(2n)) why it is not equal to tan((pi/180) * (pi/(2n))) ....please help me in this:)

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    9 months ago, # ^ |
      Vote: I like it +27 Vote: I do not like it

    Here is how to solve C1 (C2 is a whole other story). It requires you to know a little bit of trigonometry (sin, cos, tan formulas)

    Take a look at the above image, what we are looking for is the blue line. If we find the length of the line we can double it and that is our answer.

    How to do that: We can find the angle x by dividing 360/n where n is the number of vertices. Now we need to divide that by two so we can have half the x angle, lets call v = (360/n)/2 = 360/(2*n)

    We managed to create an imaginary right triangle and we know one angle (v) and the opposite side (with length 1/2 = 0.5), notice that the blue line is the adjacent to the angle v.

    Tan formula: tan(v) = opposite/adjacent For our case we solve for the adjacent and we have adjacent = opposite/tan(v) = 0.5/tan(v)

    So i hope that now it is clear that our answer is 2*(0.5/tan(360/(2*n))), just remember to double the n value at the beginning because we look for a 2n-agon.

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      9 months ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      I saw a bunch of submissions of c1 by doing some sort of summation of cosine of angles. what was that all about?

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

        Some people solved both C1 and C2 with the same code and made the same submission. I guess that C2 requires some addition. I didn't manage to solve it but I think that the solution can be broken down to finding two different sides that sum up to the total square side. (or at least that was my approach)

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          9 months ago, # ^ |
            Vote: I like it 0 Vote: I do not like it

          So C1 and C2 are not suitable for pupils(I mean students of primary schools). :(

          Luckily I find out the solution :D

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      9 months ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      why can't we use sin() to calculate directly please explain. My approach was same i just used sin, so that sin(x) = side(0.5)/radius hence radius radius = side(0.5)/sin(x).

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        9 months ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        Because sin = opposite/hypotenuse in our case we don't need the hypotenuse but the adjacent. You could use both sin and cos to factor out hypotenuse and solve for the adjacent (because tan = sin/cos) but is simpler to use the tan formula instead.

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          9 months ago, # ^ |
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          ohh there i go wrong i was actually taking hypotaneous. thank u very much for correcting me.

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          9 months ago, # ^ |
          ← Rev. 11   Vote: I like it 0 Vote: I do not like it

          .

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            9 months ago, # ^ |
              Vote: I like it 0 Vote: I do not like it

            By using sine, we will get the circumcircle radius right?

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

              .

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              9 months ago, # ^ |
                Vote: I like it 0 Vote: I do not like it

              yes, somehow I got messed up with diameter of circumcircle and side of square. haha

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      9 months ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      Thanks for the explanation.

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9 months ago, # |
  Vote: I like it +120 Vote: I do not like it

What's the point of problem C?

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

    Twice the apothem of the polygon is the side of the square [C1] https://en.wikipedia.org/wiki/Apothem

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    9 months ago, # ^ |
      Vote: I like it +17 Vote: I do not like it

    It seems C1 is without rotation, and C2 has a slight rotation, but I didn't manage to find out by how much

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      9 months ago, # ^ |
        Vote: I like it +6 Vote: I do not like it

      Same with you, and for C1 I even dont know how to get the accurate answer, I just tried and tried and finally get the answer which I even dont know why.

      For C2, I just continued to try, but unfortunately, I didnt get the answer. I find some rotation might be correct but seems it is just my guess not the answer.

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    9 months ago, # ^ |
      Vote: I like it +33 Vote: I do not like it

    Mathforcse /cy/qiang

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9 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Why does lazy propogation give TLE/MLE for problem D.

80500739

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    9 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Memory limit is 28mb.

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      9 months ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      How can this problem be solved using segment tree WTF?

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        9 months ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        A classic segment tree uses 4*N memory (I've heard this can be tightened, but I use the 4*N version). So you'll keep a leaf for every possible value in the set, representing the count of elements in the set with that value. Then you build the ST saving the sum of children. So, when you are asked to add an element K, you can do that in O(log(N)) adding 1 to every node in the path from leaf to root. If you are asked to remove the Kth element, you make a "binary search" over the segment tree. This is, if you are in node A:

        *Is the left leaf enough to find at least K elements? That means to check if ST[A*2] >= k. If it is, then you'll recursively find the Kth element in that branch *Is the left leaf not enough? Then you go to the right leaf, recursively, but the call will be query(A*2+1, k-ST[A]) because the left leaf will cover part of the "prefix" you are looking for, so you need to subtract it from K.

        There's a base case, when A is a leaf, you simply return the value associated to that leaf. Once you've found that value, you also need to update the ST by substracting 1 to every node in the path from leaf to root.

        So you see every operation is log(N)

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        9 months ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        without using lazy propogation. https://codeforces.com/contest/1354/submission/80515718

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