Hello, Codeforces!

I am happy to invite you to Codeforces Round 761 (Div. 2), which will take place on Dec/16/2021 16:35 (Moscow time). The round will be rated for participants with rating lower than 2100. **Notice the unusual starting time.**

All the problems were authored and prepared by me. The round wouldn’t be possible without these people:

- Artyom123 for excellent coordination and translation of statements.
- KAN for help with round preparation.
- arvindf232, physics0523, SecondThread, Igorbunov, kevinxiehk, generic_placeholder_name, Kotehok3, minhcool, anthony123, Elison, shishyando, SlavicG, Absyarka, erniepsycholone, dbsbs, lomienyeet for testing and valuable feedback.
- Again, arvindf232 and generic_placeholder_name for providing solutions to a problem that are better than the author’s solution.
- Again, kevinxiehk for submitting many different solutions to a problem.
- MikeMirzayanov for great Codeforces and Polygon platforms.
- You, for participating!

You will have **2 hours** to solve **5 problems**, one of which is divided into two subtasks. One of the problems is interactive, please see the guide of interactive problems if you are not familiar with it.

Wish you good luck and high ratings!

Here is the score distribution: $$$750-1000-1500-(2000-1000)-3000$$$.

The editorial is here: click.

**UPD:** Sorry but there is a checker bug in problem E. All submissions of problem E will be rejudged soon.

**UPD2:** Rejudge done, the round remains rated.

Congratulations to the winners (will be updated after system tests):

**Congrats!**

As a tester, I can confirm that the contest does indeed contain problems.

As a tester, I can confirm that this is indeed a contest.

Whoa, spoilers

As a tester can you tell which problem has subtasks?

yesn't

As a not tester, D1, D2.

As a tester, I can confirm that I am indeed a tester.

why does this comment has more upvotes, than post itself?(

Because they deserve it as a tester

Well, coz he is SecondThread!

Because maybe people like me are waiting to give votes on the post after the contest.

Because it is indeed a comment.

ok the word

`problems`

here is super vague, I hope it does not contain problems in the problems :DAs a nontester I hope you are not scamming us.

As a participant, I can confirm that I am gonna participate.

As a tester, I cannot participate. :(

As a tester, I would recommend everyone to read all problems

Good luck and have fun :D

As a tester, snowysecret orz

I agree with you!!!!!

Orzzzzzzzzzzzzz

As a tester, I can reassure you the tasks are pog. Highly recommend everyone to register. I can also confirm that the first question is QA.

As a tester, I can say the problem statements are short, so I recommend everyone to join!

I like short statements <3

I enjoy doing tasks with short statements also :D

As a tester, I sometimes test CF rounds.

Hopefully interactive problem isn't divided into subtasks.

Great prediction!

Looking forward to tomorrow's round! Finally a 5-problem div2 :)

6 if you consider subtasks though

yet another unusual time

Unusually usual unusual time

Why does this make sense? XD

Normally, one would expect “unusual time” to usually be unusual, yet, if you look at the times of some of the recent contests, it is actually pretty usual, at least more usual than what one would usually expect, which is unusual. Thus, the unusual time can be considered to be unusually usual.

make sense

As a reader, I can see that there are 3 times more testers than problems. :)

blue forces

how easy it is to increase contribution "as a tester";

You know ,How difficult it is to test a problem "as a tester"?

it's time to become cyan (tyan)

You should have become tyan(cyan) long ago, good luck.

Suprisingly, the contest is set to the Kazakhstan's Independence Day(16 Dec). Moreover, it is 30th anniversary of our independence.

GL&HF!

would love it if Problem E is the nice interactive problem without subtasks.

After a long time, finally an interactive problem !

As a participant, I am excited to participate in the contest XD.

as a participate ,i can confirm that i will lose my rating

you have no rating to lose xD

as a participate ,,i can confirm to you that you are not funny XD

A legendary grandmaster can say the same thing to you too ...

go play with your div3

Okay play div2 with me then and give them some time to get better bruh ,but don't bully mate:)

I assure you, bullying is bad for your health..

What's the rule of contests proposing now? IIRC I think only orange or author in previous rounds can propose contests now, it this rule changed again? >_<

I dmed the admins from Codeforces and they let me do it.

How long has your round been in queue for?

answering this question might cause problems :D

Around five months.

Congratulations on your first CF round as a problem setter. Hoping for a good round and some good problems.

No disrespect intended, but I'm very curious to how a mid-low expert can come up with problems that sufficiently challenge a 2100 rated person. I guess I'll find out tomorrow.

Rating, apart from estimating one's skills and knowledge, at the very least also heavily depends on speed and ability to handle stress.

Look up Danny Sleator, he has a cyan rating here FYI, but I bet you wouldn't doubt his knowledge or ability to create a problemset that'd easily make the vast majority of competitors look pathetic.

"Knowing" or "being able to" is not enough, you need speed, this is also why this thing we're doing is called

competitiveprogramming. Good luck tomorrow!Given enough time, (some) experts can solve hard (<=2600R) problems and when you make the problem you have plenty of time to think of a solution for it before proposing. Also, when people usually make problems they have a vague idea of a possible solution in mind, which makes solving it way easier compared to a person who just read the problem and has to solve it in a 2h period.

Guess you've found out now:)

Small question: If there is anyone who proposed a round in the past, how long is it from when you last set it public to the day it got approved? Is it 3 months or longer?

Hope I can reach expert after this round :D

so nitin_05 and ashokesen02 both registered in this round.

Get a life. Every time i see you spamming these names. I am not supporting cheating but you can not stop someone from cheating.

As a beginner I can confirm that the contest is gonna be hard for me. But I am hopeful.

Why?

Because I participated in division 3 contest yesterday. But I couldn't solve any problems.

This is not important. You get better little by little.

I wish good luck everyone!According to the distribution: $$$ 750−1000−1500−(2000−1000)−3000 $$$, is D1 harder than D2?

Can someone please explain then?

I can't be sure but I think if someone is able to solve D1, a small optimization will solve D2 too. So coming up with an approach for D1 will be difficult than solving D2 after you have solved D1.

But I would suggest don't make any assumptions from this score distribution, check number of solves during the contest.

You can think of a question D with 3000 points split into two parts.

As a jia xin tang,ran ran ni dai wo zou ba!

Jiamen!please take me away

Is the problem score related to the difficulty level of the problem?

In most cases, yes

It's time to become purple !!!

Let me get to the blues(^.^) UPD: Missed by 1 rating points, Sed life

Can anyone explain what does this line mean — "The jury is adaptive, which means the indices of impostors may not be fixed beforehand and can depend on your questions. It is guaranteed that there is at least one set of impostors which fulfills the constraints and the answers to your questions at any time."? Does this mean that the indices may change as we ask questions?

You must ask in "ask a question" section, for any doubts during the contest time.

Thanks. Wasn't aware of that :)

I think it means test case are different base on your query.

As you know, in codeforces, blue is stronger than red.

I have a question: If I didn't register on the round, didn't send ny codes, but during the contest took a glance at the results board and the problems, my rating won't change right? I'm just too worried

Rating won't change unless you make a submission

Unless you didn't send any code on the contest, there will be no change in your rating. I remember registering on some contests and I didn't send any code, but my rating hadn't changed.

Ok, thank you very much

.

[removed]

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You can assume that c = 1, then bruteforce all possible a and b, you will find answer quickly, because lcm(1, 2, ... 100) is much bigger than 1e9

Nice problem set!

Yeah, really nice, top 10 worst div.2 I've seen.

A: Boring implementation.

B: Classical Problem, I've seen it >5 times on cf.

C: The only common problem.

D: It seems that the interactor is a joke, and the problem is a joke as well.

E: Maybe to hard for div.2E, my 2800 friend can't solve it as well

The observations used in 2E are not advanced at all, and is very cool for its position. So it's not too hard for div2. Last problems of div2 typically get 50 solves which is the case here too.

OK I read the editorial, now I'm more embarrassing

I solved E. It's quite interesting. How did your friend get 2800?

Any hints for C? I had some thoughts with greedy algorithms but I always can find counterexamples. Can it be solved greedily? Or at least what's the main idea?...

You’re not supposed to ask for hints about the problems during the contest, you can read the rules for more information. If you really want to know how to solve the problem, you can wait for the editorial for the contest, which will be out soon;)

Editorial is out, check it out!

Try printing n % i for 1 <= i <= n and see if you can make an observation

Yes, it can be solved greedily.

awesome problem E! [tho TL is a bit tight]

Going to be

BlueAgain.. :DNice Problemset Sir !! Thank you for the contest <3

I wasted 30 minutes on D1 because I had mistakenly assumed that

`0`

meant crewmates and`1`

meant imposters.I am not just a clown. I am the entire circus.

Damn bro, me too. Can I join the circus with you?

wasted 40 min and managed to figure out in the last 30 seconds of the contest

third impostorHint for B.

:)

The entire contest was casework...

Great round! I like the problems!

I told you before the contest XD, like even if its not the easiest div-2 rounds, the tasks are still very interesting and elegant (Go read editorial)

Problem D (especially D2) is really really nice.

The $$$\frac{4n}{3} + 6$$$ and $$$n + 6$$$ observations are really wow moments when you get them.

Unfortunately I got stuck on $$$\frac{4n}{3}$$$ to $$$n$$$ optimization for too long to code it before the contest ended T_T.

Thank you for the contest! I like it a lot~~

E is really nice

( >︹<) AMOGUS ( >︹<) IM JUST IRONIC BTW ( >︹<) IM NOT ACTUALLY AMONG US NORMIE

Really curious to know how the adaptive interactor for problem D1 and D2 works?

I think the main principle of the adaptive interactor is to fail any solution that asked wrong queries and cannot figure out the answer surely. I don't know what algorithms are used, but if it detects it can fail the solution, it will find the imposter sequence and do it.

As a tester, I can give some insights:

Unfortunately, there isn't any protocol we know of that definitely eliminates random solutions. The interactor here relies on a combination of anti-random heuristics and exhaustively trying small tests to root out random solutions.

How to solve D?The editorial is already posted at https://codeforces.com/blog/entry/97920

Very boring game!!!

The observation of D2 is pretty good, but after that it is just boring implementation. Glad I am just above div2 so I don't have to force myself to write it down.

For B problem , my solution is wrong , why?

Firstly , I enumerate the factors of $$$n$$$ , assuming the factor of $$$n$$$ is $$$x$$$.

Assuming that $$$a = μx , b = λx , c = x$$$ , and $$$ \gcd(λ,μ) = 1$$$. So this is accord with the second condition.

Then through the first condition , we know that $$$μx + λx + x = n$$$

so $$$μ + λ = \frac{n}{x}-1$$$

so when $$$(\frac{n}{x}-1)$$$ is ODD ， we let $$$μ=\lfloor(\frac{n}{x}-1) / 2\rfloor$$$ , $$$λ = μ+1$$$ , then we got the final answer , which is $$$O(T \sqrt{n})$$$

WHY THIS IS WRONG ANSWER ? PLEASE HELP ME.

sorry for my poor english!

Assume n = 15. Divisors of 15 are x = 1, x = 3, x = 15 and x = 5. All of them are odd, so n/x will always be odd too. So, n/x-1 will be even. You haven't described this case if I understood you right.

SpoilerApart from the tricky problem statement in D, very nice contest! Kudos to the creator!

Thank you for the nice problems!

The jury is adaptive, which means the indices of impostors may not be fixed beforehand and can depend on your questions.So if you have n = 6 and ask queries as in D1 editorial (1, 2, 3), (2, 3, 4), (3, 4, 5), (4, 5, 6), couldn't the imposters just always go to the positions the query asks, so there all the answers to the queries will be same?

"It is guaranteed that there is at least one set of impostors which fulfills the constraints and the answers to your questions at any time."

This means that there must be a set of indices consistent with all your queries so far.

So for all your queries to return imposters, there must be imposters as the majority in the positions of all queries, for which the smallest set we can do is $$${2, 3, 4, 5}$$$ which violates $$$\frac{n}{3} \lt k \lt \frac{2n}{3}$$$. So its impossible to generate a counter-case under the given constraints which satisfies all your queries.

ooh, thanks. my brain is kinda dead from waking up at 6am for advent of code lol.

great contest. I'm lucky that after I can't get idea of D1, I checked E and finished it in time.

( >︹<) AMOGUS ( >︹<) IM JUST IRONIC BTW ( >︹<) IM NOT ACTUALLY AMONG US NORMIE

For D2, why the limit is n+6? My submission with total<=n+8 got WA and I spent long time making it to total<=n+4.

Why the limit so tight？I think n+20 it better.

If you really want the big constant factor got WA. Why not let the limit = n + 4，or you are stupid that you can't do with limit = n + 4?

Really good contest. Statements were clear and problems were interesting. Loved it!

any tips for interactive problems i become blank as soon as i saw them and dont know how to approach is there any good video resources if so plz share thnx in advance.

Firstly, try to read the announcements before every contest. It was mentioned there that one of the problems is interactive along with a link to understand what it was and how to approach it. U can go to that link for understanding interactive problems.

Can anyone suggest me some problems like today's C?

https://csacademy.com/ieeextreme15/task/appointment/

I don't know why I wa1 on problem D. So sad.

Your queries can contain same players in them. You can try 6 players with 345 as impostors for example.

Nice contest! Although I will be getting a negative delta, still I liked the contest.

Problemset was awesome !

What a cool contest snowysecret made for today. There would be more such wonderful contests. Most of all I liked problems D1 and D2. Although unlikely I would solve D2. But the D1 problem was quite solvable for me. But of course it's a pity that I could'n solve it. And problem C was also cool. In one sentence, I would like to say, thank you snowysecret for such a wonderful contest.

can someone please tell why this is giving me RTE on the first test case? it is working fine on my compiler https://codeforces.com/contest/1617/submission/139558421

In queries t1,t2,t3, a can be equal to b+1 or vice versa, then in that case query(a,a+1,b) or similar queries will be wrong as all indices won't be distinct.

nice problems , had fun in this contest

What's the meaning of "Wrong question format" in D2, I WA many time on it;

Some possibilities that can cause "Wrong question format":

The first character is not

`?`

or`!`

The first character is

`?`

, and the three indices outputted are not distinct, or not in the range $$$[1, n]$$$C can be solved in linear time 139510468

nyaharo