Hi, Codeforces!

Educational Codeforces Round #1 will take place on 13 ноября 2015 года в 18:00 MSK for the first and second divisions. You can read about educational rounds here.

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 five 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.

Educational rounds are (and would) be prepared by me, Edvard Davtyan from Saratov SU Daemons team. Problems was invented by me and MikeMirzayanov. Thanks to my teammate danilka.pro for testing problems and MikeMirzayanov for Codeforces, Polygon and idea of the educational rounds.

I hope you will enjoy the problems. If they will be too simple for you, you are welcome to the second educational round, problems there should be harder.

Good luck and have fun!

UPD: The first phase of round is over. I remind that the results is not final and you can hack any solution during the day.

UPD2: Please use only deterministic generators. For example you should not use srand(time(NULL)) with calling rand() function in C++. Your generator should always generate the same test.

Hi everybody!

First, I invite you to take part in an official Testing Round 12. The fact is, the Codeforces team has made numerous changes to the platform (details are below), and we want to be sure that the basic functionality remained unchanged. This round will have a shortened duration of 1.5 hours, consist of 3 (maybe 4) problems that you might have already seen before. Its purpose is to test the system on the one hand, and on the other hand — to brighten up a Wednesday evening. Of course, the round will be unrated.

Now the main thing. This coming Friday (yes, the 13th of November) Codeforces starts another line of rounds. We called them Educational Rounds. Using my students at the Saratov State University Programmings Competitions Training Center as an example, I regularly notice that even those who have a considerable progress in the results on the rounds often have narrow purview in terms of standard topics and ideas, they are not familiar with many well-known problems and methods. The fact is, the rounds often avoid any folk or classical subjects, thus underdeveloping the purview of young generation of the participants.

We are pleased to announce the start of a series of educational rounds! They will take place with the regularity of a round 2-4 times per month.

Their characteristics are like that:

• The duration is classic — 1.5 — 2.5 hours;
• The goal is rather to practice and to educate, than to compete;
• Not only problems, but also exercises can be used;
• Useful, even well-known ideas will be reused in order to introduce them to a wide range of participants;
• Often the formal text of the statements;
• Unrated (perhaps for now);
• We will try to conduct them in the ACM-ICPC mode (if there are long waiting lists, we may change the approach);
• The results that are obtained after the end of the round, are preliminary;
• After the end of the round will be a 24-hour period of open hacks — any visitor of Codeforces may try to hack any complete solution to a problem of the last round (either from a contest, or from practice), the source code of hacking solution is available (you can copy the text and, for example, stress it);
• All successful hacks from the previous item will be added to the official test set and after as long as 24 hours after the end of the round retesting of all complete solutions will be made;
• Only after the final standings based on improved test data, the results are final;
• The results of a round are calculated separately by division;
• Our ability to process such problems are limited, so actually the test suites from the jury are expected to end up incomplete — we are looking forward to your hacks!

Basically, we will be oriented towards the members of the second division, but often these rounds will be of interest to more experienced participants.

For now preparing problems to these rounds will be concentrated at the Saratov State University Programming Competitions Training Center, most of the work with the problems will be accomplished by Edvard Edvard Davtyan. We wish him good luck, enthusiasm and energy!

See you at the Testing Round 12, and later at the Educational Codeforces Round 1.

Good time of day, Codeforces!

I am glad to announce that this Sunday, 8th November at 19:30 MSK, Codeforces Round #330 for both divisions will take place.

The problemset of the round has been prepared for you with pleasure by Alex fcspartakm Frolov and me, Dan Sagunov. We want to thank the coordinator of Codeforces Gleb GlebsHP Evstropov for his valuable help in problem preparation, Mike MikeMirzayanov Mirzayanov for Codeforces and Polygon systems, Maria Delinur Belova for translating problem statements into English and Vladislav winger Isenbaev and Alex AlexFetisov Fetisov who have tested the round problemset.

Every participant will be given two hours to solve five problems. We have tried to make the round problemset variative and interesting and therefore we strongly recommend to read all the problem statements during the round. Scoring will be announced later as always.

We wish good luck and high rating to everyone!

UPD. We are sorry again for the mistake in Cdiv2/Adiv1 problem: jury's solution is working wrong in odd n case. We are hoping that other problems was (or will be such in upsolving) interesting and useful.

Anyway, let's congratulate the round winners:

First division winners:

1. jcvb
2. 2222
3. KAN

second division winners:

1. Tagrimar
2. fsps60312
3. uhateme

Editorial could be found here.

UPD. Problem Cdiv2/Adiv1 was fixed and now it has the statement and solution which jury meant it to be. Problem has been returned to the contest, so feel free to upsolve it.

Hello, Codeforces!

I'd like to invite you to Codeforces Round #329 (Div. 2). It'll be held on Wednesday, November 4 at 19:30 MSK and as usual Div. 1 participants can join out of competition.

Writers are Stanislav Naumov (josdas) and Roman Korobkov (romanasa). There will be 5 problems and you'll have 2 hours to solve them. I hope you enjoy the problems.

Great thanks to GlebsHP (Gleb Evstropov) for helping me preparing the contest, to Delinur (Maria Belova) for translating the statements into English, to MikeMirzayanov (Mike Mirzayanov) for the Codeforces and Polygon.

The scoring distribution will be announced later.

Good luck and high rating!

UPD: The round will use the dynamic scoring with 250 points step.

UPD2: Editorial

Codeforces Round #328 (Div. 2) will take place on October 31, 19:30 MSK, as usual Div. 1 participants can join out of competition.

Problem Setter: Morphy (Alei Reyes)

Coordinator: GlebsHP (Gleb Evstropov)

English to Russian translator: Delinur (Maria Belova)

Codeforces and Polygon: MikeMirzayanov (Mike Mirzayanov)

Hope you enjoy the problem set.

The score distribution will be announced later.

UPD. Score Distribution: 500 — 1000 — 1500 — 2000 — 3000

UPD. Problem Analysis is available

Congrats to the winners!

Div. 2

shamir0xe

lbn187

SuperLoser

Div. 1

uwi

NanoApe

Haghani

Starting from October 2015 ratings formulas are open. They are given in the post. It is likely that from time to time we will change them slightly, it will be reflected here.

The basic idea of Codeforces rating system is to generalize Elo rating to support games with multiple participants.

Each community member is characterized by value r_i — integer number. Roughly speaking, the higher value means better results in the contests. Rating is calculated/recalculated so that the equality strives to be correct:

where P_i, j is probability that the i-th participant has better result than the j-th participant. Therefore for two participants the probability to win/lose depends on subtraction of their ratings. For example, if the difference of ratings is equal to 200 then stronger participant will win with probability ~0.75. If the difference of ratings is equal to 400 then stronger participant will win with probability ~0.9.

After a contest the values r_i change in a way to satisfy main formula better.

Let’s calculate expected place seed_i for each participant before contest. It equals to the sum over all other participants of probabilities to win (to have better place than) the i-th plus one because of 1-based place indices:

For example, before Codeforces Round 318 [RussianCodeCup Thanks-Round] (Div. 1) tourist had rating 3503 and his seed was ~1.7, and Petr had rating 3029 and expected place ~10.7.

General idea is to increase r_i if actual place is better than seed_i and to decrease r_i if actual place is worse than seed_i.

Having seed_i and actual place, let’s calculate their geometric mean m_i. You can think about it as an something average between seed_i and actual place shifted to the better place. Using binary search find such rating value R which the i-th participant should have to have a seed_i = m_i. Obviously the rating r_i should be modified to become closer to R. We use d_i = (R - r_i) / 2 as rating change for the i-th participant.

It's almost all except the phase of fighting against inflation. Inflation works as follows: the rich get richer. We will try to avoid it. If we assume that the rating was already calculated fair (i.e. everybody has perfect statistically based rating) then expected change of rating after a contest is equal to zero for any participant.

Choose a group of the most rated (before the round) participants and decide that their total rating shouldn’t change. We use heuristic value as a size of such group. Let’s find the sum of d_i over participants from group and adjust all values d_i (for all participants) to make the sum to be zero. In other words, r_i = r_i + inc, where inc =  - sum_s / s, sum_s is sum of d_i over s participants from chosen group.

After the round 327 we restricted the effect in following way. Firstly, we do r_i = r_i + inc, where inc =  - sum(d_i) / n - 1, sum(d_i) is sum of all d_i. It makes the sum of all d_i to be near zero and non-positive in the same time. Secondly, we apply idea from the previous paragraph, but inc = min(max( - sum_s / s,  - 10), 0). Thus, the effect of modification can not reduce rating for more than ~10 points.

By the way, for any consistent rating the following assertions should be true:

• if the participant A had worse rating than the participant B before the contest and finished the contest on the worse place then after recalculations the the rating of A can’t be greater than the rating of B
• if A finished the contest better than B but A had worse rating before the contest then A should have equal or greater rating change than B.

In particular, formulas are tested to satisfy the both items on each ratings recalculation.

You may read the actual Codeforces code to recalculate ratings here: 13861109.

Hello, dear participants of Codeforces community!

Starting from Codeforces Round #327 I'll be the new coordinator and will manage preparation process for all regular Codeforces rounds and other contests held on this platform. I'll do my best to increase the quality of contests we offer you, though Zlobober already raised this bar high. Let's cheer him once again, for all the great job he had done!

Tomorrow round will be held using the problems of Moscow team olympiad for high-school students. Do not be tricked by the word "school" — there will be a gold medalist of IOI 2015 participating and some candidates to this year Russian IOI team, meaning the level of the problems will be appropriate. I am sure everyone will find an interesting problem to enjoy.

The problemset was prepared by the team of Moscow authors (list is incomplete): Zlobober, romanandreev, meshanya, wilwell, glebushka98, timgaripov, thefacetakt, haku, LHiC, Timus, Sender, sankear, iskhakovt, andrewgark, ipavlov, StopKran, AleX leaded by your humble servant GlebsHP and the chairman of the jury Helen Andreeva.

Special thanks to Delinur for translation and stella_marine for correction.

Five problems will be offered in both divisions and the scoring distribution will be announced later (good traditions should live).

UPD. Round time. Please note that Russia is not changing the timezone this night, while about 100 countries do so. Be sure to check when round starts in your timezone!

UPD2. Please note, that the distribution motivates you to read all the problems! Div1.: 750-1000-1250-1750-2500 Div2.: 500-1000-1750-2000-2250

UPD3. The results and the editorial will be published later, after the closing ceremony of the official competition.

UPD4. The system testing is over, results are now final. Feel free to upsolve and read other contestants code. Congratulations to Div. 1 top-5:

1. Endagorion
2. JoeyWheeler
3. sdya
4. RAD
5. -XraY-

UPD5. Problem analysis is now available.