Hello, Codeforces!

Educational Codeforces Round 14 will take place on 13 July 2016 at 19:00 MSK for the first and the second divisions.

<I already have a long list of problems, so don't be upset if your problems wasn't used yet>

The round will be unrated for all users and it will be held with extented ACM ICPC rules. You will have two hours to solve six problems. After that you will have one day to hack any solution you want. You will have access to copy any solution and test it locally.

If you have ideas for some problems or maybe already prepared problems that you can't use in rounds or official competitions, you can write to me.

It seems that it is harder to invent interesting simple problems (like A and B) than difficult ones. So don't be afraid to suggest interesting simple or very simple tasks (of course the difficult tasks are also needed). Please send only the problems for which you know the solution with clear statement (the story is not required). And also please add one or two samples to make the problem statement more clear.

Thanks a lot to them and all others who are sending the problems! The number of unused problems are increasing. If I didn't miss anything I already replied to all who sent me the problems at least 5-6 days ago. Please be patient if your problem was not used a long while.

</I already have a long list of problems, so don't be upset if your problems wasn't used yet>

The problemset was suggested by Codeforces users. The problem А was suggested and prepared by user Arthur Jaworski KingArthur. The problem B was suggested by Nikita Melnikov nickmeller. The problem C was suggested by user blowUpTheStonySilence. The problem D and E was suggested by Zi Song Yeoh zscoder (he is on IMO now, so good luck to him). The problem F was suggested Michael Kirsche mkirsche.

As I said the problem A is prepared by Arthur Jaworski KingArthur. All other problems was prepared by me (Edvard Davtyan). Thanks to Tatiana Semyonova Tatiana_S for checking the English statements. The problems was tested by users suggested them, respectively: Arthur Jaworski KingArthur, Nikita Melnikov nickmeller, user blowUpTheStonySilence and Michael Kirsche mkirsche. Thanks a lot to all of them!

I hope you will enjoy the problems! I think the problemset is balanced and not difficult.

Good luck and have fun!

UPD 1: All the hacks for the problem D will be rejudged.

UPD 2: Hack phase will be extended until tomorrow.

UPD 3: Sorry for the problems in the problem D. The solutions that got WA3 were rejudged. The problem is OK now.

UPD 4: The contest is finished. All the solutions were judged on the full testsets. The editorial will be published soon.

UPD 5: The editorial is published.

Hello CodeForces! I'd like to invite you to the online mirror of an open championship of Switzerland called the Helvetic Coding Contest.

The Helvetic Coding Contest is a yearly contest held at the EPFL in Lausanne by the PolyProg association. The contest itself took place in March, but the online mirror is scheduled on Sunday, 10th of July, 11:00 Moscow time. The duration is 4:30.

Rules:

• you can participate in teams or individually (1-3 people),

• standard ACM-ICPC rules (no hacking),

• the contest is not rated,

• if you have participated in the onsite contest, please do not participate in the mirror.

You will help the cow Heidi protect humanity against a zombie apocalypse. The contest will feature 6 series of 2-3 related tasks with increasing difficulty (say easy/medium/hard). Sometimes it may be the case that a solution for the hard version solves all of them, but usually not. In the onsite contest, teams could only access the medium version of a problem once they have solved the easy, and so on; in the mirror, there is no such constraint and you will be able to see all versions since the beginning. (Otherwise, the formats of the onsite and the mirror are the same.) We think that the problemset is diverse and interesting, and while the contest is ACM-style, you will find that some problems are far from standard :)

We promise to publish a very nice editorial as soon as the contest ends.

Acknowledgments: I had the pleasure of coordinating the team of problemsetters for this contest: gawry, Christian Kauth, maksay, boba5551, DamianS and myself. Thanks also go out to people who helped with the statements and testing: Jeremy Rabasco, Valerian Rousset, Sjlver, Wajeb; GlebsHP for Russian translations and CodeForces coordination, as well as everyone involved in the actual onsite contest, who are too many to name here. We also thank the sponsors Open Systems and AdNovum. Lastly, thanks to MikeMirzayanov for CodeForces and Polygon (which was used to prepare the problems).

Finally, in a bit of autopromotion, note that you can use Hightail to automatically test your solutions :) Good luck!

After-contest update:

• congratulations to the winners:

1. Zg: gustav, ikatanic

2. Endagorion

3. squark_tutan_RR: ngfam_kongu, I_love_Hoang_Yen, s-quark

4. mehlxneh: AntiForest, JoeyWheeler, xxTastyHypeBeast666xx

5. FTP++: pwypeanut, jacobtpl, zhangguangxuan99

6. Команда, в которой непростые в...ку с максимальным рейтингом: Um_nik, kb., Tinsane

7. Khodaro Shokr: SeyedParsa, PrinceOfPersia

8. Coder

• the editorial is available (PDF); feel free to ask questions here,

• hope you enjoyed the contest!

Hi everyone!

I want to present you the Codeforces Round #361 (Div. 2) that will take place on the 6th of July at 19:35 MSK. The contest was prepared by Gabriel I_Love_Tina Cojocaru and Mihail ThatMathGuy Tarigradschi.

I'd like to thank Dan danilka.pro Sagunov and Gleb GlebsHP Evstropov for their help in preparing the round and for making the round possible,Mike MikeMirzayanov Mirzayanov for the great Codeforces and Polygon platforms.

Good luck in the contest!!

UPD1.In this round you will help our hero Mike solve tasks he encounters every day.

UPD2. Editorial

UPD3.Congratulations to the winners of the round.

Div 1:

1.Um_nik

2.cgy4ever

3.anta

4.HellKitsune

5.ksun48

6.waterfall

7.Kaban-5

8.I_love_Tanya_Romanova

9.I_love_tigersugar

10.RomeoFantastik

Div 2:

1.Sky4teen

2.TudorMiclovan

3.darius.marian

4.Hirasawa_Yui

5.Chloe_fan

6.AnonymousBunny

7.fenchen

8.Grandpa

9.6eJIa9IzZzTeHb

10.raiders7

Hi!

We're glad to invite you all to participate in Codeforces Round #360(Div. 1 and Div. 2) which will take place on Wednesday, 29 June, 20:05 Moscow time. Check it in your timezone!

The problems are designed by Man (Parsa Abdollahi) and me. It's our first Codeforces round and we hope you enjoy competing it as much as we enjoyed preparing it! (^◡^)

Our special thank goes to JeBeK (Peyman Jabbarzade) who helped us a lot in preparing and testing the round. Many thanks to GlebsHP (Gleb Evstropov) for his help in preparing the contest, and MikeMirzayanov (Mike Mirzayanov) for great platforms Polygon and Codeforces. We also want to thank Zlobober (Max Akhmedov) for testing our round.

We wish (and expect!) you all many Accepted solutions! ( ﾟ▽ﾟ)/

UPD: Problems are going to be about Pari and Arya.

UPD2 Congratulations to the winners!

Div. 1:

1. jqdai0815
2. tourist
3. Egor
4. xyz111
5. subscriber
6. riadwaw
7. ainta
8. jcvb
9. Um_nik
10. Shik

Div. 2:

1. Julek
2. polygonia
3. Snipx
4. I_love_littlechild
5. AminAnvari
6. Shayan
7. yashkumar18
8. Archies
9. Clone3
10. lature

Editorial + some challenges will be published soon.

UPD3: The editorial is out!

Ladies and Gentlemen!

The Codeforces round #359 for both divisions will happen on 23 June at 7:35 PM MSK. We have been choosing and solving problems for a long time and we hope that they will seem quite interesting.

All of the problems were created under guidance of Endagorion (Mikhail Tikhomirov) by Moscow high school graduates: cdkrot (Dmitry Sayutin), ch_egor (Egor Chunaev), themikemikovi4 (Mikhail Sorokin) and me. It is our second Codeforces Div.1 + Div.2 round.

Thanks to coordinator GlebsHP (Gleb Evstropov) for help with preparing problems and to MikeMirzayanov (Mikhail Mirzayanov) for excellent systems Polygon and Codeforces. Also, the round is possible only because of Endagorion's help.

Participants will be given five problems in both divisions. The scoring distribution will be announced later.

Good luck to all!

UPD: All statements are based on the fairy tale "The Snow Queen" by H. C. Andersen.

UPD2: The scoring distribution:

Div. 1: 500-1250-1250-2000-2500

Div. 2: 500-1000-1500-2250-2250

UPD3: Congratulations to the winners!

Div. 1:

1. Petr

2. jcvb

3. dotorya

4. ainta

5. matthew99

6. Errichto

7. yosupo

8. Myungwoo

9. RAVEman

10. zemen

Div. 2:

1. aasddf

2. jupanul

3. ItsLastDay

4. dacin21

5. RedRiver

6. --d

7. heracle

8. abcdeedcba

9. IgorKoval

10. 131131yhx

UPD4:

Cheating detection may affect the ratings a little.

UPD5:

The editorial can be found here.

The Codeforces Marathon Round 1 is over (results comment). As the final solutions are being tested, I think many participants will want to share ideas and learn alternative approaches. I'll start with the ideas I tried myself, both successful and not; as I've seen in the submissions, the contestants have more ideas, but I hope they will share themselves. For each of the solutions below, the numbers in square brackets are minimum, mean and maximum score when running on 1000 tests locally. I have to note in advance that the constants and technicalities in the solutions are not considered optimal: the score just shows approximate relation between ideas and may often be improved a bit.

Hello!

Let us remind you that Russian Code Cup 2016 Elimination Round will take place on June 19, 2016 at 14-00 Moscow time. Top 200 from each qualification round can take part in Elimination round, 200 best coders in Elimination round will get branded championship t-shirt, and top 50 will advance to the Final Round that will take place in September. There are money prizes to grab in the Final Round.

Good luck everyone and see you at http://russiancodecup.ru !

There has been talk about this already, but in case anyone missed it:

The 2016 edition of Internet Problem Solving Contest is going to take place today (starting time).

It's a 5-hour contest for teams of up to 3 people, but there's also an individual division, so feel free to participate even if you can't find yourself a team at this point.

There's going to be an ACM-like number of problems (12+), ranging from classic algorithmic problems to quite unusual ones. Most problems have an easy subproblem worth 1 point and a hard one worth 2 points (think CodeJam); ties are broken using ACM rules (sum of times).

The practice session is over. The contest is over!

Read the rules or my previous blogposts about this for more info.

Belated, yet necessary warning!

Since this is a 5-hour contest where you can execute codes locally, some inputs will be YUGE (gigabytes). Accordingly, they will have fairly high system requirements. Get a good computer. While the official solutions quite comfortably run on my mediocre laptop, if you write something too inefficient, you can encounter a nasty surprise, e.g. frozen system. It happened to me last year.

If an input is big, you won't have to download it; instead, there will be a generator (typically a Python script, meaning they aren't very fast). It's usually a good idea to run all generators as early as possible — as long as it doesn't slow down your computer too much, you can avoid a situation where you can't submit a problem because the contest ended before your generator.

Actually, you should just try to read as many problems as possible and determine your strategy after you can guess your chances well enough.

Some quick stats

11145 submissions
5370 successful submissions (48% success rate)
797 active teams out of 1376 (58% did not give up before even trying)
773 teams with positive score (97% of active teams)
12/13 problems solved by someone
maximum number of fully solved problems: 10/13
lowest non-zero success rate: D (D2: 20%)
highest success rate: C,F (C2,F2: 85%)

highest success rate (easy subproblems): G1 (85%)
lowest success rate (easy subproblems): J1,M1 (25%)

hard problems (<50 teams solved) sorted by difficulty:
E: 0/13
M: 2/10
J: 4/17
H: 11/17
B: 11/18
L: 16/46
K: 40/107