By 1-gon, history, 3 months ago, In English

¡Buenos días! (That's Spanish for "what's up homies")

On Dec/06/2020 17:35 (Moscow time) we will host Codeforces Global Round 12.

It is the sixth round of a 2020 series of Codeforces Global Rounds. The rounds are open and rated for everybody.

The prizes for this round:

  • 30 best participants get a t-shirt.
  • 20 t-shirts are randomly distributed among those with ranks between 31 and 500, inclusive.

The prizes for the 6-round series in 2020:

  • In each round top-100 participants get points according to the table.
  • The final result for each participant is equal to the sum of points he gets in the four rounds he placed the highest.
  • The best 20 participants over all series get sweatshirts and place certificates.

Thanks to XTX, which in 2020 supported the global rounds initiative!

The problems were written and prepared by smart Cuban Devil and stupid Americans fivefourthreeone and 1-gon.

We would like to distribute our thanks equally to the following people who made this round possible.

You will have 3 hours to solve 8 problems (and 2 subtasks). If you want to lose rating, then we encourage you not to read all the problems.

May rating be distributed from each according to his ability, to each according to his needs!

UPD: Here's the score distribution. Good luck, have fun!


UPD: Hope you enjoyed the problems! Editorial is posted.

UPD: System testing finished, congrats to the winners!

  1. Benq
  2. tourist
  3. jiangly
  4. izone
  5. ecnerwala
  6. Um_nik
  7. ksun48
  8. kefaa2
  9. maroonrk
  10. yosupo

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Announcement of Codeforces Global Round 12
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By KAN, 3 months ago, In English

As you probably know, most of the rounds on Codeforces are created by active community members. We are very grateful to our talented authors from all around the world for bringing their ideas to tens of thousands of competitors, providing fun and learning experience for contestants of every expertise, from beginners to the world's top programmers. This post will cover the steps you need to take propose a round and become a problem writer on Codeforces.

Who can be a problem author

Best problems usually come from the most experienced participants. We have limited resources for rounds reviewing, so to make the process flawless for writers, starting January 1st, 2021, the following requirements for problem authors will be applied:

  • rating: orange or above, or
  • was a writer in the past.

The restrictions may be updated in the future, the post will be updated accordingly. If your round is already in the review status, don't worry, it will be reviewed regardless of the current restrictions.

If you are an experienced writer/jury member in other competitions, please write any current coordinator (listed below) a private message about your wish to prepare a contest and describe your experience in this area.

How to propose a round

If you meet the requirements, you should have the link "propose a contest/problems" in the personal menu on the right sidebar. Click on the link and follow the instructions carefully. Prepare and submit your proposal for a contest and problems.

In some time you will have a coordinator(s) assigned to your contest proposal. They will review your problems and you will discuss the problems with them. Please be active, respond to coordinators' questions in time, make the proposed changes quickly. Codeforces coordinators are very experienced participants and reviewers. Please respect their opinion, and note that they have the highest priority in any decisions on the problems.

Our coordinators are: antontrygubO_o, isaf27, 300iq, BledDest, adedalic, Nebuchadnezzar and KAN.

You can only propose a whole problemset for a Div 2 or Div 1 + Div 2 round. We don't consider single problems. Also, we don't consider proposals for Div 3/4 or Educational rounds.

How to prepare problems

Problems for Codeforces rounds are prepared in Polygon. It is not recommended to prepare problems before the coordinator's approval, especially if it's the first time you propose a contest.

For each problem you will need to write a clear statement (in English or Russian), code a validator (a program that checks tests' correctness), a checker if necessary (a program that checks output), and of course correct solution. Also, you will need to create tests to check the contestants' solutions on. Your coordinator will do the statement translation if necessary, check your work, and suggests what to change and add if needed.

Before you start working on problems, even if you have experience working with Polygon, you must read these rules. All the work with checkers, validators, interactors, and generators is done with testlib.h library, you can read the introduction by the link and download examples from Polygon. Whenever you have any troubles and/or difficulties, contact your coordinator for help. It is much faster to do something right from the first time, then to do it wrong and then redo it again.

Writers reward

It is interesting and challenging work to invent and prepare problems but is also time-consuming and demanding work. That's why we pay authors for preparing problems for Codeforces. The rewards are:

Round Type $
Global* $800
Div 1 + Div 2 $400
Div 2 $200

We reserve the right to reduce a reward by $50 if something in the round goes wrong because of the authors' fault.

*Global rounds are selected by coordinators from available proposals.

Best Div 1 + Div 2 rounds can be selected to be sponsored, in this case, the authors' reward is higher and is discussed separately.


We hope that with your help we will keep the regular stream of regular rounds full of fresh and interesting problems! If you have any questions on how to become a problem writer, ask in the comments. Please note that unrelated or outdated comments in this blog will be cleared once in a while to keep the blog and comments up-to-date.

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By djm03178, history, 3 months ago, In English

오랜만이에요, 코드포스! (Long time no see, Codeforces!)

I'd like to welcome all of you to Codeforces Round #688 (Div. 2)! The contest will start at Dec/04/2020 16:05 (Moscow time), and it is rated for all participants with ratings under 2100. Note the semi-unusual start time.

You will be given 6 problems and 2 hours and 15 minutes to solve them. The score distribution will be announced soon.

All problems are prepared by me, with a lot of help from the testers making me realize that my solutions are dumb.

Thanks to Green55, JooDdae, cs71107, YeongTree, DreamingLeaf, jh05013, Lemonade255, 39dll, alswhp, Ku_Top, sonjaewon, slah007, jooncco, and rkm62 for testing the round, and especially xiaowuc1 for helping polish English statements as well. I would also like to thank 300iq for round coordination, and MikeMirzayanov for the great Codeforces and Polygon system.

See you in the round!

UPD: The scoring distribution is 500 — 1000 — 1500 — 2000 — 2500 — 3500.

UPD 2: The round is finished. Thanks for your participation! I'm sorry about underestimating the difficulty of problem B, but I hope you still enjoyed the problems! The editorial will be posted in a minute.

UPD 3: The editorial is out!

UPD 4: Congratulations to the winners!

Div. 2

1: caoyizhong

2: Depth_First_Search

3: Misaka23334

4: TheWolfWhoCriedBoy

5: EzioAuditoreDaFirenze

Unofficial Div. 1

1: Geothermal

2: jiangly

3: neal

4: saketh

5: Pyqe

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By awoo, history, 3 months ago, translation, In English

Hello Codeforces!

On Nov/30/2020 17:35 (Moscow time) Educational Codeforces Round 99 (Rated for Div. 2) will start.

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.

Good luck to all the participants!

Congratulations to the winners:

Rank Competitor Problems Solved Penalty
1 neal 7 223
2 jiangly 7 224
3 tute7627 6 190
4 nocriz 6 195
5 noimi 6 213

Congratulations to the best hackers:

Rank Competitor Hack Count
1 MarcosK 41:-6
2 dapingguo8 34:-2
3 jerdno 12:-4
4 ARTpositive 10:-1
5 halyavin 9:-1
357 successful hacks and 769 unsuccessful hacks were made in total!

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

Problem Competitor Penalty
A edickLPM 0:00
B SSRS_ 0:03
C corol 0:03
D pajenegod 0:08
E neal 0:29
F jiangly 0:55
G jiangly 1:17

UPD: Editorial is out

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By JetBrains, history, 3 months ago, In English

Hello Codeforces!

One of the most memorable Kotlin rounds of this year took place just recently. More than 6.7 thousand participants have registered for it. We thank everyone for your interest in Kotlin! Everyone who took part in this round will receive a link to download a personalized online certificate within the next two weeks. Work on this has already started.

Our warm congratulations to the winners of the 1st, 2nd, and 3rd place, who receive cash prizes:

  1. tourist
  2. Benq
  3. Geothermal

A Kotlin Heroes t-shirt and exclusive Kotlin stickers will be presented to:

Top 50
  1. tourist
  2. Benq
  3. Geothermal
  4. natsugiri
  5. Golovanov399
  6. eatmore
  7. xiaowuc1
  8. Smoge
  9. TadijaSebez
  10. Bench0310
  11. ainta
  12. azukun
  13. uwi
  14. wu_qing
  15. SYury
  16. Tlatoani
  17. Lily
  18. majk
  19. Potassium
  20. Heltion
  21. pashka
  22. Fortin
  23. htkb
  24. Aleks5d
  25. Shanks
  26. IgorSmirnov
  27. lucifer1004
  28. tob123
  29. fjuengermann
  30. pparys
  31. shilov
  32. dim3x38on
  33. Spheniscine
  34. Maksim1744
  35. Quang
  36. gafeol
  37. golions
  38. WiwiHo
  39. PugachAG
  40. ynq1242
  41. jonas.havelka.42
  42. yudedako
  43. gleb.astashkin
  44. Dart-Xeyter
  45. Ji_Kuai
  46. EmK
  47. timmyfeng
  48. receed
  49. Son
  50. Nikitosh

And also these participants won branded T-shirts:

List of 50 random winners
  1. pseuda
  2. Hakiobo
  3. 0solve
  4. Musin
  5. Moririn2528
  6. pye
  7. ndrsh
  8. jetoyrico
  9. __v0id
  10. TToPoCeHoK
  11. manish.17
  12. Koala13
  13. htamop
  14. Dimitrys
  15. JNU_dh_AK_WF
  16. keertan
  17. jcarlson212
  18. da_louis
  19. smit.mangukiya
  20. ShSergey
  21. eneskayiklik
  22. ali.gh2236
  23. dubina
  24. Lius
  25. ScrappyDoo
  26. shubhammitt
  27. ambershogun
  28. alaza5
  29. IdeaSeeker
  30. Soul_Full_Of_Thunder
  31. ekeitaro
  32. Amir_Reza
  33. Exiti
  34. The_Uncrowned_King
  35. reLLike
  36. VPurice
  37. B0rnToLose
  38. j00sko
  39. fkrihkl
  40. Protego_totalum
  41. haru_mizuki
  42. iff.julian
  43. BMking
  44. bansalraghav_19
  45. dvd1
  46. stein_code
  47. ErimKerim
  48. Niloy.
  49. steph.dubedat
  50. algomath

Finally, the last, but not least, gift for today: ICPC-experience, an invitation to the Moscow World Finals 2021, all-inclusive on-site (hotel, meals, ceremonies, and swag, are included; visa, flights, transportation to the contest location is not).

And a special recognition prize from the ICPC goes to tourist.

If you are still new to Kotlin you could look into a tutorial on competitive programming in Kotlin and watch these videos on competitive programming in Kotlin. Also, all the previous Kotlin Heroes Rounds are available for practice here: Episode 1, Episode 2, Episode 3, Episode 4 and Episode 5: ICPC Round.

We wish you all the best and hope you enjoy Kotlin.

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By KAN, 3 months ago, translation, In English

Hi all!

This weekend, at Nov/29/2020 10:05 (Moscow time) we will hold Codeforces Round 687. It is based on problems of Technocup 2021 Elimination Round 2 that will be held at the same time.

Technocup is a major olympiad for Russian-speaking high-school students, so if you fall into this category, please register at Technocup 2021 website and take part in the Elimination Round.

Div. 1 and Div.2 editions are open and rated for everyone. Register and enjoy the contests!

The problems authors are; amethyst0, eidan, Diegogrc, bensonlzl, Maripium, antontrygubO_o, and KAN. Thanks to antontrygubO_o and 300iq for their help in coordination. Also huge thanks to testers for their invaluable help: Golovanov399, rkm62, golions, dantrag, _cherry_, gigabuffoon, Andres_Alumets, firiexp, growup974, Nero, GGMU, K0u1e, Bench0310, dorijanlendvaj, Um_nik, thenymphsofdelphi, Merkurev, kokokostya, wucstdio, smile_boi, abhishhh1!

Have fun!

Thank you for participation! We hope you enjoyed the problems. Congratulations to winners!

Technocup 2021 - Elimination Round 2

  1. Kirill22
  2. fastmath
  3. VEGAnn
  4. alexxela12345
  5. solver777

Codeforces Round #687 (Div. 1, based on Technocup 2021 Elimination Round 2)

  1. ecnerwala
  2. ainta
  3. conflict
  4. al13n
  5. kort0n

Codeforces Round #687 (Div. 2, based on Technocup 2021 Elimination Round 2)

  1. gang_leader
  2. iLoveIOI
  3. Linqi05
  4. xaohu
  5. detect

Upd: Rating updates published.

UPD2: The editorial is published.

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By vovuh, history, 3 months ago, translation, In English

Hello! Codeforces Round #686 (Div. 3) will start at Nov/24/2020 17:35 (Moscow time). You will be offered 6 or 7 problems (or 8) with expected difficulties to compose an interesting competition for participants with ratings up to 1600. However, all of you who wish to take part and have rating 1600 or higher, can register for the round unofficially.

The round will be hosted by rules of educational rounds (extended ACM-ICPC). Thus, during the round, solutions will be judged on preliminary tests, and after the round it will be a 12-hour phase of open hacks. I tried to make strong tests — just like you will be upset if many solutions fail after the contest is over.

You will be given 6 or 7 (or 8) problems and 2 hours to solve them.

Note that the penalty for the wrong submission in this round (and the following Div. 3 rounds) is 10 minutes.

Remember that only the trusted participants of the third division will be included in the official standings table. As it is written by link, this is a compulsory measure for combating unsporting behavior. To qualify as a trusted participants of the third division, you must:

  • take part in at least two rated rounds (and solve at least one problem in each of them),
  • do not have a point of 1900 or higher in the rating.

Regardless of whether you are a trusted participant of the third division or not, if your rating is less than 1600, then the round will be rated for you.

Thanks to MikeMirzayanov for the platform, help with ideas for problems and for coordination of my work. Thanks to my good friends Daria nooinenoojno Stepanova, Mikhail awoo Piklyaev, Maksim Neon Mescheryakov and Ivan BledDest Androsov for help in round preparation and testing the round. Also thanks to Artem Rox Plotkin and Dmitrii _overrated_ Umnov for the discussion of ideas and testing the round!

Good luck!

UPD: I would also like to thank Ivan Gassa Kazmenko for invaluable help with the round preparation!

UPD2: Editorial is published!

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