Can anyone help me understand this problem:

http://codeforces.com/problemset/problem/930/B

I mean can’t vasya ask all letters(which will give probability 1) ?

I know i am wrong,Please if someone could clarify the question.

Thanks in advance!!

Edit:Sorry to reopen it.But i am now having difficulty in understanding the approach for the question,also cant understand the editorial.And also can someone explain how the answer for string: bbaabaabbb is 0.1?