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".