CF676C.Vasya and String

High school student Vasya got a string of length $n$ as a birthday present. This string consists of letters 'a' and 'b' only. Vasya denotes beauty of the string as the maximum length of a substring (consecutive subsequence) consisting of equal letters.

Vasya can change no more than $k$ characters of the original string. What is the maximum beauty of the string he can achieve?

The first line of the input contains two integers $n$ and $k$ ( $1<=n<=100000,0<=k<=n$ ) — the length of the string and the maximum number of characters to change.

The second line contains the string, consisting of letters 'a' and 'b' only.

Print the only integer — the maximum beauty of the string Vasya can achieve by changing no more than $k$ characters.

In the first sample, Vasya can obtain both strings "aaaa" and "bbbb".

In the second sample, the optimal answer is obtained with the string "aaaaabaa" or with the string "aabaaaaa".