Hello everybody.

I would like to invite you to participate in a new competition, that will be held on the 30th of October at 16:00 MSK. It will be a part of the series of winter programming school olympiads, but it will be only a standart Codeforces round for adult participants.

Teams aren't invited, only single members. The official rules will be ACM-ICPC.
Jury has decided to increase the duration of the competition, which will be 4 hours long.

Attention: People, who are considered out of competition, are participants of Codeforces Round #38, others are participants of School Personal Contest #1. But we will use merged result table for updating rating.

The authors are me, Mike Mirzayanov and Artem Rakhov. Also I'm expressing gratitude to Maria Belova, Max Ivanov and Natalia Bondarenko for help.

P.S. Please, don't forget to register to the competition!

A. Extra-terrestrial Intelligence

This task is very easy, and I hope all of you have solved it. But let's talk about it.

Let input sequence is named as a, and its length is n. Then let's save sequence x₁, ..., x_k is increasing order - all of positions, where a_xi = '1'.

We must check if this sequence x₁, ..., x_k is a arithmetic progression. We save variable d = x₂ - x₁ and check:
for all 1 ≤ i < k, is "d = x_i + 1 - x_i" correct.

If it's correct, output "YES", else "NO".

Welcome to Codeforces Beta Round #19.

Authors of today's contest are Artem Rakhov and me. Thanks to Mike Mirzayanov, Edvard Davtyan and Julia Satushina for help in the organisation.

I hope, you will have fun.

Good luck!

P.S. After start of the contest, you can download the statements:

English

Russian

UPD. The contest has finished and you can see the standings and tasks. The winner and only participant who has solved all the problems is kalinov. Congratulations!