### dbsic211's blog

By dbsic211, 10 months ago,

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:

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!

• +773

 » 10 months ago, # |   +449 As a tester, I can confirm that the contest does indeed contain problems.
•  » » 10 months ago, # ^ |   +347 As a tester, I can confirm that this is indeed a contest.
•  » » » 10 months ago, # ^ |   +225 Whoa, spoilers
•  » » » » 10 months ago, # ^ |   +32 As a tester can you tell which problem has subtasks?
•  » » » » » 10 months ago, # ^ |   +68 yesn't
•  » » » » » 10 months ago, # ^ | ← Rev. 2 →   0 Someone
•  » » » » » 10 months ago, # ^ |   +24 As a not tester, D1, D2.
•  » » » 10 months ago, # ^ |   +197 As a tester, I can confirm that I am indeed a tester.
•  » » 10 months ago, # ^ |   +24 why does this comment has more upvotes, than post itself?(
•  » » » 10 months ago, # ^ |   -54 Because they deserve it as a tester
•  » » » 10 months ago, # ^ |   +28 Well, coz he is SecondThread!
•  » » » 10 months ago, # ^ | ← Rev. 2 →   +26 Because maybe people like me are waiting to give votes on the post after the contest.
•  » » » 10 months ago, # ^ |   +2 Because it is indeed a comment.
•  » » 10 months ago, # ^ |   +64 ok the word problems here is super vague, I hope it does not contain problems in the problems :D
•  » » 10 months ago, # ^ |   0 Man you got jokes.
•  » » 10 months ago, # ^ |   0 As a nontester I hope you are not scamming us.
•  » » 10 months ago, # ^ |   +5 As a participant, I can confirm that I am gonna participate.
 » 10 months ago, # |   +165 As a tester, I cannot participate. :(
 » 10 months ago, # |   +50 As a tester, I would recommend everyone to read all problemsGood luck and have fun :D
 » 10 months ago, # |   +67 As a tester, dbsic211 orz
•  » » 10 months ago, # ^ |   +32 I agree with you!!!!!
•  » » » 10 months ago, # ^ |   0 Orzzzzzzzzzzzzz
 » 10 months ago, # | ← Rev. 3 →   +41 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.
 » 10 months ago, # |   +65 As a tester, I can say the problem statements are short, so I recommend everyone to join!
•  » » 10 months ago, # ^ |   +7 I like short statements <3
•  » » » 10 months ago, # ^ |   +10 I enjoy doing tasks with short statements also :D
 » 10 months ago, # |   +182 As a tester, I sometimes test CF rounds.
 » 10 months ago, # |   -16 Hopefully interactive problem isn't divided into subtasks.
•  » » 10 months ago, # ^ |   +5 Great prediction!
 » 10 months ago, # |   +7 Looking forward to tomorrow's round! Finally a 5-problem div2 :)
•  » » 10 months ago, # ^ |   +5 6 if you consider subtasks though
 » 10 months ago, # |   -34 yet another unusual time
•  » » 10 months ago, # ^ |   +80 Unusually usual unusual time
•  » » » 10 months ago, # ^ |   +10 Why does this make sense? XD
•  » » » » 10 months ago, # ^ |   +33 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.
•  » » » » » 10 months ago, # ^ |   +8 make sense
 » 10 months ago, # |   +49 As a reader, I can see that there are 3 times more testers than problems. :)
 » 10 months ago, # |   +26 blue forces
 » 10 months ago, # |   -26 how easy it is to increase contribution "as a tester";
•  » » 10 months ago, # ^ |   +24 You know ,How difficult it is to test a problem "as a tester"?
•  » » » 10 months ago, # ^ | ← Rev. 3 →   -33 i think testing a question is much more diffcult than solving the question. why are people downvoting without actually understanding the context. i mean to say its easy to get upvote if you are tester
 » 10 months ago, # | ← Rev. 3 →   -36 16 december is the victory day of my country Bangladesh. Hope this day will be good with this contest.
•  » » 10 months ago, # ^ |   -16 Why this comment get downvoted. Well i can guess the country of downvoters.
 » 10 months ago, # |   +2 it's time to become cyan (tyan)
•  » » 10 months ago, # ^ |   0 You should have become tyan(cyan) long ago, good luck.
 » 10 months ago, # |   0 Suprisingly, the contest is set to the Kazakhstan's Independence Day(16 Dec). Moreover, it is 30th anniversary of our independence.GL&HF!
•  » » 10 months ago, # ^ |   -6 Hey, Happy Independence Day. It's our (Bangladesh's) victory day too.
 » 10 months ago, # |   -10 would love it if Problem E is the nice interactive problem without subtasks.
 » 10 months ago, # |   0 Hope to perform well in the contest. All the best to all.
 » 10 months ago, # |   0 hoping for specialist in this contest
 » 10 months ago, # |   -8 After a long time, finally an interactive problem !
 » 10 months ago, # |   +3 As a participant, I am excited to participate in the contest XD.
 » 10 months ago, # |   +8 as a participate ,i can confirm that i will lose my rating
•  » » 10 months ago, # ^ |   0 you have no rating to lose xD
•  » » » 10 months ago, # ^ |   +24 as a participate ,,i can confirm to you that you are not funny XD
•  » » » » 10 months ago, # ^ |   0 You'll get green this time inshaallah.
•  » » » 10 months ago, # ^ |   +17 A legendary grandmaster can say the same thing to you too ...
•  » » » » 10 months ago, # ^ |   -40 go play with your div3
•  » » » » » 10 months ago, # ^ |   +22 Okay play div2 with me then and give them some time to get better bruh ,but don't bully mate:)
•  » » » 10 months ago, # ^ |   0 I assure you, bullying is bad for your health..
 » 10 months ago, # |   +39 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? >_<
•  » » 10 months ago, # ^ |   +41 I dmed the admins from Codeforces and they let me do it.
•  » » » 10 months ago, # ^ |   +30 How long has your round been in queue for?
•  » » » » 10 months ago, # ^ |   0 answering this question might cause problems :D
•  » » » » 10 months ago, # ^ |   +54 Around five months.
 » 10 months ago, # |   +8 Congratulations on your first CF round as a problem setter. Hoping for a good round and some good problems.
 » 10 months ago, # |   -9 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.
•  » » 10 months ago, # ^ |   +61 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 competitive programming. Good luck tomorrow!
•  » » 10 months ago, # ^ |   +44 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.
•  » » 10 months ago, # ^ |   +5 Guess you've found out now:)
 » 10 months ago, # |   +4 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?
 » 10 months ago, # | ← Rev. 2 →   +1 Hope I can reach expert after this round :D
 » 10 months ago, # |   0 Thanks for guide of interactive problems
 » 10 months ago, # |   -10 Agree
 » 10 months ago, # |   +14 so nitin_05 and ashokesen02 both registered in this round. will they cheat again in this round? if so, will they not be caught and be rated again?! who will cheat better and drain more ratings from other innocent users?!?
•  » » 10 months ago, # ^ |   +3 Get a life. Every time i see you spamming these names. I am not supporting cheating but you can not stop someone from cheating.
•  » » » 10 months ago, # ^ |   0 hi Nitin Singh Rajput
 » 10 months ago, # |   -8 As a beginner I can confirm that the contest is gonna be hard for me. But I am hopeful.
•  » » 10 months ago, # ^ |   0 Why?
•  » » » 10 months ago, # ^ |   0 Because I participated in division 3 contest yesterday. But I couldn't solve any problems.
•  » » » » 10 months ago, # ^ |   +1 This is not important. You get better little by little.
•  » » » » » 9 months ago, # ^ |   0 Thank you.
 » 10 months ago, # |   0 horraayy
 » 10 months ago, # |   +2 I wish good luck everyone!
 » 10 months ago, # |   0 As a non-tester, why are all testers so cool ?
 » 10 months ago, # |   -12 According to the distribution: $750−1000−1500−(2000−1000)−3000$, is D1 harder than D2?
•  » » 10 months ago, # ^ |   0 no
•  » » 10 months ago, # ^ |   0 Can someone please explain then?
•  » » » 10 months ago, # ^ |   +13 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.
•  » » » 10 months ago, # ^ |   +11 You can think of a question D with 3000 points split into two parts.
•  » » » 10 months ago, # ^ |   0 D will be interactive
•  » » » » 10 months ago, # ^ |   0 C
 » 10 months ago, # |   -31 As a jia xin tang,ran ran ni dai wo zou ba!
•  » » 10 months ago, # ^ |   -11 Jiamen!please take me away
 » 10 months ago, # |   0 Is the problem score related to the difficulty level of the problem?
•  » » 10 months ago, # ^ |   0 In most cases, yes
•  » » » 10 months ago, # ^ |   0 Thanks for your answer.
 » 10 months ago, # |   0 It's time to become purple !!!
 » 10 months ago, # | ← Rev. 2 →   0 Let me get to the blues(^.^) UPD: Missed by 1 rating points, Sed life
 » 10 months ago, # |   +5 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?
•  » » 10 months ago, # ^ |   +9 You must ask in "ask a question" section, for any doubts during the contest time.
•  » » » 10 months ago, # ^ |   0 Thanks. Wasn't aware of that :)
•  » » 10 months ago, # ^ |   0 I think it means test case are different base on your query.
 » 10 months ago, # |   +49 As you know, in codeforces, blue is stronger than red.
 » 10 months ago, # |   0 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
•  » » 10 months ago, # ^ |   +15 Rating won't change unless you make a submission
•  » » 10 months ago, # ^ |   +1 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.
•  » » » 10 months ago, # ^ |   0 Ok, thank you very much
 » 10 months ago, # |   -61 I coudn't figure out how to solve task B properly, what to do if $n$ is a prime number? Can I somehow find answer greedily? I decided to iterate over some values, but still convinced it's gonna be WE or TLE on system tests. 139521778, probably it will work, but I don't really understand if that's the case.
•  » » 10 months ago, # ^ | ← Rev. 2 →   -15 [removed]
•  » » » 10 months ago, # ^ | ← Rev. 2 →   -16 That was reckless from me to post it right now, yep. By the way, can someone look into my submission if they didn't solve the problem? Or if didn't lock the problem?
•  » » 10 months ago, # ^ |   +1 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
 » 10 months ago, # |   0 Nice problem set!
•  » » 10 months ago, # ^ | ← Rev. 2 →   -95 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
•  » » » 10 months ago, # ^ |   +30 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.
•  » » » » 10 months ago, # ^ |   +17 OK I read the editorial, now I'm more embarrassing
•  » » » 10 months ago, # ^ |   +50 I solved E. It's quite interesting. How did your friend get 2800?
 » 10 months ago, # |   -51 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?...
•  » » 10 months ago, # ^ |   +1 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;)
•  » » » 10 months ago, # ^ |   +33 Editorial is out, check it out!
•  » » 10 months ago, # ^ |   +1 Try printing n % i for 1 <= i <= n and see if you can make an observation
•  » » 10 months ago, # ^ |   0 Yes, it can be solved greedily.
 » 10 months ago, # |   +12 awesome problem E! [tho TL is a bit tight]
 » 10 months ago, # |   +2 Going to be Blue Again.. :DNice Problemset Sir !! Thank you for the contest <3
 » 10 months ago, # | ← Rev. 2 →   +77 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.
•  » » 10 months ago, # ^ |   +6 Damn bro, me too. Can I join the circus with you?
•  » » 10 months ago, # ^ |   0 wasted 40 min and managed to figure out in the last 30 seconds of the contest
•  » » 10 months ago, # ^ |   0 third impostor
 » 10 months ago, # |   0 Hint for B.:)
•  » » 10 months ago, # ^ |   0 If n is even, the u can split them into 3 even integers. If n is odd. Then u can split them into 3 odd integers
•  » » 10 months ago, # ^ |   0 C can always be 1, just bruteforce a = n — 3, b = 2, while(__gcd(a, b) != 1 and (a != b)) a++,b--; worked for me!
 » 10 months ago, # |   0 The entire contest was casework...
 » 10 months ago, # |   +4 Great round! I like the problems!
•  » » 10 months ago, # ^ |   +8 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)
 » 10 months ago, # |   +36 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.
 » 10 months ago, # |   -11 As not a tester , I find that I have forgotten the start time XD
 » 10 months ago, # |   +1 Thank you for the contest! I like it a lot~~
 » 10 months ago, # |   0 E is really nice
 » 10 months ago, # |   +17 ( >︹<) AMOGUS ( >︹<) IM JUST IRONIC BTW ( >︹<) IM NOT ACTUALLY AMONG US NORMIE
 » 10 months ago, # |   +8 Really curious to know how the adaptive interactor for problem D1 and D2 works?
•  » » 10 months ago, # ^ |   +8 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.
•  » » » 10 months ago, # ^ |   +8 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.
 » 10 months ago, # |   0 How to solve D?
•  » » 10 months ago, # ^ | ← Rev. 2 →   0 The editorial is already posted at https://codeforces.com/blog/entry/97920
 » 10 months ago, # |   -28 Very boring game!!!
 » 10 months ago, # |   +19 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.
 » 10 months ago, # | ← Rev. 3 →   0 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!
•  » » 10 months ago, # ^ | ← Rev. 2 →   0 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.
•  » » » 10 months ago, # ^ |   0 Oh, you are right , in this case my algorithm is wrong. Thanks!
 » 10 months ago, # |   +7 Spoiler
 » 10 months ago, # |   0 Apart from the tricky problem statement in D, very nice contest! Kudos to the creator!
 » 10 months ago, # |   +2 Thank you for the nice problems!
 » 10 months ago, # |   0 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?
•  » » 10 months ago, # ^ |   +1 "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.
•  » » » 10 months ago, # ^ |   0 ooh, thanks. my brain is kinda dead from waking up at 6am for advent of code lol.
 » 10 months ago, # |   0 great contest. I'm lucky that after I can't get idea of D1, I checked E and finished it in time.
 » 10 months ago, # |   0 ( >︹<) AMOGUS ( >︹<) IM JUST IRONIC BTW ( >︹<) IM NOT ACTUALLY AMONG US NORMIE
 » 10 months ago, # |   -57 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?
 » 10 months ago, # |   +4 Really good contest. Statements were clear and problems were interesting. Loved it!
 » 10 months ago, # |   +1 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.
•  » » 10 months ago, # ^ |   +3 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.
 » 10 months ago, # |   0 Can anyone suggest me some problems like today's C?
•  » » 10 months ago, # ^ |   0
•  » » » 10 months ago, # ^ |   0 page not found.
 » 10 months ago, # |   +1 I don't know why I wa1 on problem D. So sad.
•  » » 10 months ago, # ^ |   +3 Your queries can contain same players in them. You can try 6 players with 345 as impostors for example.
 » 10 months ago, # |   0 Nice contest! Although I will be getting a negative delta, still I liked the contest.
 » 10 months ago, # |   0 Today i got to know that unlike codechef codeforces accepts the last correct solution instead of the best and lost some points:(...
 » 10 months ago, # |   0 Problemset was awesome !
 » 10 months ago, # |   +42 What a cool contest dbsic211 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 dbsic211 for such a wonderful contest.
 » 10 months ago, # |   0 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
•  » » 10 months ago, # ^ |   0 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.
 » 10 months ago, # |   0 nice problems , had fun in this contest
 » 10 months ago, # |   0 What's the meaning of "Wrong question format" in D2, I WA many time on it;
•  » » 10 months ago, # ^ |   0 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 $[1, n]$
 » 10 months ago, # |   0 Excuse me? Problem E ，Why timeout test6 mycode: https://codeforces.com/contest/1617/submission/139606135