Hi!

On Tuesday, July 11, 2017, at 16:35 UTC Codeforces Round #423 will be held. Some of the problems will be from VK Cup 2017 Finals, some are prepared by Codeforces team to complete the problemset.

Note that there will be two rounds based on VK Cup Finals, so I want to ask finalists not to discuss problems until the end of the second round.

There will be six problems in each division, four of them shared. The problems of this round are proposed, prepared and tested by: vintage_Vlad_Makeev, demon1999, fcspartakm, Perforator, MikeMirzayanov, PavelKunyavskiy, qwerty787788, Belonogov, izban, tourist, vepifanov, AlexFetisov, winger, Errichto, Gassa, naagi, ashmelev, Endagorion, ifsmirnov, Arterm and me. Huge thanks to all who helped with the preparation!

There will be prizes from VK social network in this round! Namely, among participants solved five or more problems in second division, or three or more problems in first division, five are to be selected randomly. They will receive championship souvenirs. There is no country nor language restriction, everyone can win a prize. One don't have to have participated in VK Cup to receive the prize. Exact selection algorithm will be announced before the start of the round. There will be prizes in the second round using Finals' problems as well!

Good luck!

Congratulations to the winners!

Div. 1:

1. W4yneb0t
2. jqdai0815
3. ko_osaga
4. Radewoosh
5. dotorya

Div. 2:

1. ccz181078
2. _rqy
3. houruize
4. _Is_It_Rated_
5. liutianyi000

The analysis is here.

Corresponding problems from VK Cup 2017 - Finals:

• Problem 827B - High Load is problem 823A - High Load, the author is MikeMirzayanov;
• Problem 827C - DNA Evolution is problem 823B - DNA Evolution, the authors are MikeMirzayanov and me;
• Problem 827E - Rusty String is problem 823D - Rusty String, the authors are vintage_Vlad_Makeev and demon1999;
• Problem 827F - Dirty Arkady's Kitchen is problem 823G - Dirty Arkady's Kitchen, the authors are demon1999 and vintage_Vlad_Makeev.

The souvenirs winners are:

Congratulations!

Today, on July 9, 2017 the VK Cup 2017 Finals is being hold in Saint Petersburg!

Just to remind you, the total prize fund is 2490368 rubles, 20 teams consisting of 39 contestants in total will fight for it. Past years champions are tourist and niyaznigmatul in 2015, and tourist and subscriber in 2016. Who is going to take the first place today?

You can watch live results of the competition here, the start is at 08:05 UTC.

Good luck to all contestants and fans!

Final standings

UPD: Hooray! The results of the final competition were summed up. Congratulations to the winners!

1. ershov.stanislav, aid,
2. V--o_o--V, LHiC,
3. enot110, I_love_Tanya_Romanova,
4. zemen, Zlobober,
5. nikgaevoy, tranquility,
6. RomaWhite, witua,
7. Trumen, felix,
8. Rubanenko, MrDindows,
9. Tima, akim,
10. Golovanov399, -imc-,
11. Goofy57, Shapo,
12. ztk, ballon,
13. Vladyslav, Omelianenko,
14. Seemann, bezhimr,
15. Temirulan, Aidos.
16. EvErka, greencis,
17. IlyaLos, danilka.pro,
18. YakutovDmitriy, budalnik,
19. Lilith, Copymaster,
20. SergeiFedorov.

Hello everyone!

This Sunday July 2, 19:05 MSK rated Codeforces Round #422 (Div. 2) will be held.

This time I'm the only one author in contradistinction to the Round 415. Many thanks to coordinator Alex netman Vistyazh for help in preparations of the round, Vladislav winger Isenbaev, Alex AlexFetisov Fetisov, Nikolay KAN Kalinin and Ildar 300iq Gainullin for testing the problems and also to Mike MikeMirzayanov Mirzayanov for Codeforces and Polygon platforms.

As in the Round 415, the main characters of the tasks will be the stunning girl Noora and hacker Leha.

Participants will be given six problems and two hours to solve them. As always, participants from the first division can take part out of competition. Scoring will be announced before the round.

I hope you'll enjoy my round. I wish everyone good luck and high rating!

UPD1: Note that the number of tasks which would be proposed for the solution has been changed.

UPD2: The scoring for this round is: 500 — 750 — 1250 — 1750 — 2250 — 2500.

UPD3: Congratulations to the winners! Editorial will be posted soon.

Div. 2:

1. Nisiyama_Suzune

2. cephian

3. Cherish

4. Irina_Rog

5. ColdSu

Div. 1:

1. natsugiri

2. Um_nik

3. KrK

4. irkstepanov

5. _Reborn_

Special congratulations to natsugiri, who is the only one among all the participants who coped with all six tasks!

UPD4: Editorial.

Hello Codeforces!

On June 29, 18:05 MSK Educational Codeforces Round 24 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.

The round will be unrated for all users and will be held on extented ACM ICPC rules. After the end of the contest you will have one day to hack any solution you want. You will have access to copy any solution and test it locally.

You will be given 7 problems and 2 hours 15 minutes to solve them.

The problems were prepared by Mikhail awoo Piklyaev, Alexey Perforator Ripinen and me.

Good luck to all participants!

I also have a message from Harbour.Space University about the ML course held at the university:

Nowadays machine learning technologies are widely used in practice in various applied fields such as retail, mass media, PR and marketing, banking, telecommunications, manufacturing, science and many other areas.

We are pleased to announce our Industrial Machine Learning course, aimed to teach the structure and the life cycle of the machine learning process and covers topics ranging from the problem statement to final quality assessment as well as estimation of the economic effect.

This course is taught by two Data Scientists from Yandex, Emeli Dral and Victor Kantor. Emeli Dral is the Head of Predictive Analysis, whereas Victor Kantor is the Head of User Behaviour Analysis Group at Yandex.

http://in.harbour.space/data-science/industrial-machine-learning/

They share with us their knowledge on everything from the most promising applications of data science today to applying the latest advancements in machine learning to your work.

UPD: The editorial can be found here.

During the discussion after the Round 421 it turned out that the author's solution was wrong. Yes, we had several testers, we had a bruteforce solution that worked correctly, we had a stress testing between the bruteforce and the main solutions. However, all these measures haven't prevented the mistake: the testers haven't noted the issue, the stress haven't found any counter-tests. The solutions also produced the same answers all small tests we had in the testset, so we only found the issue when a test used for a hack was added.

We are terribly sorry about the mistake. We did our best to ensure the problems are ok, I don't know any available method that we haven't used to check the problem.

We've spent the last few hours trying to come up with a new, correct solution to the problem, but right now I don't see any solution that seems undoubtedly correct to me. There are a few that work better than the author's, but yet unproved, you can try to prove this one, for example. I think this problem should be solvable with careful case analysis, and I'm trying to do this right now. I'll post the results here. I'd also be glad if someone post the solution with proof.

Thus, I apologize that the round should be declared unrated. Hope for your understanding.

Hello, Codeforces!

I'd like to invite you to Codeforces Round #421 that will be held at June 27th, 2017 at 17:35 MSK.

Yes, it's my first round, and yes, it's another round from purple.

Great thanks to KAN for help in preparation of this round, danilka.pro for help on first stages and testing, Belonogov, WHITE2302, hloya_ygrt, Perforator for testing the problems, and MikeMirzayanov for great systems Codeforces and Polygon.

As always, participants of both divisions will be given 5 problems and 2 hours to solve them. Scoring will be announced before the round.

Good Luck and Have Fun!

UPD1: Scoring is standart for both divisions: 500-1000-1500-2000-2500

We are working on the issue with problem A in div. 1. We will announce the decision later.

Due to the issue with problem A, the round is declared unrated. More details.

Nevermind all promblems, Editorial must be, so it's here.

Hi everyone!

It's the round you've been waiting for... Codeforces Round #420 (Div 2). It will take place tomorrow, 25 June 2017, at 17:35 MSK. As always, Div. 1 participants are encouraged to join out of competition. The round will be rated for Div. 2 participants and will have 5 problems to be completed in 2 hours.

I wouldn't be able to create this round without the help of many people: KAN for lots of help in preparing the contest, 300iq and Alladdin for testing and providing feedback, and as always MikeMirzayanov for the Codeforces and Polygon platforms.

This will be my second contest, and I'm glad to get such a nice round number :). Hopefully, you'll find the problems interesting. Of course, you should read all the problems so you can solve problems you find easy or interesting.

To excite some and disappoint others, the round theme will have nothing to do with the number 420. Instead, continuing the anime themes of the last two rounds, round #420 will feature mad scientist Rintaro Okabe (don't read spoilers!) from the anime Steins;Gate. Statements will not contain any spoilers, but you might find the problems slightly more amusing if you've watched the series. Of course, you should watch the series if you havent :)!

As per Codeforces tradition, scoring distribution will be announced shortly before the contest. Good luck, and I hope you can help Okabe with all 5 tasks :D!

Scoring is standard: 500-1000-1500-2000-2500!

Update: Sorry for delays.

Update: Editorial is here: http://codeforces.net/blog/entry/52895

I really apologize for the issues during the round. It's upset a lot of people and I should have taken more care. Hopefully you won't hold this against Okabe or Steins;Gate anime :P

Here are the contest winners! Congratulations!

Div. 1

1. nuip
2. uwi
3. sugim48
4. xumingkuan
5. HellKitsune

Div. 2

1. Parachutes
2. forgive_her
3. visitWorld
4. y553546436
5. a_student_follower

Thanks for participation everyone! Hope to write more problems in the future (without bugs)!

Mabuhay!

I am honored to announce Codeforces Round #419. It will take place tomorrow, on June 17, 17:35 MSK.

The problems in this round are written by me, robinyu. I believe it is the first Codeforces round with a Philippine writer. I would like to extend my heartfelt gratitude to kevinsogo, AlexFetisov and winger for their invaluable help in testing the problems, KAN for his endless patience and support in preparing the contest, and of course MikeMirzayanov for the absolutely amazing Codeforces and Polygon platforms.

In this round, you will be helping Karen in her day-to-day routine.

There will be pretty pictures, drawn by the lovely Kazenokaze. Do not miss the round!

Both divisions will be given five problems and will have two hours to solve them. Keeping with authentic Codeforces tradition, scoring will be announced right before the round.

As a matter of course, you are highly recommended to read all the problems; though problems are arranged by expected difficulty, you may find some problems easier than others.

I hope you will enjoy the problems as much as I did writing them!

The scoring for this round is:

Div. 1: 500 — 1250 — 1500 — 2250 — 2250
Div. 2: 500 — 1000 — 1500 — 2000 — 2750

Good luck, and I wish you all high ratings and bug-free code!

Update: Congratulations to all winners!

Special congratulations to Radewoosh for his second consecutive Div. 1 round victory, and to both him and yutaka1999 for solving all tasks!

Thanks for your patience. The editorial has been posted. Hope it was worth the wait!