Special Substring

A substring of a string is a contiguous sequence of characters from the string. For example, BC is a substring of ABCD which starts from the second character of ABCD. Another example, ABC is a substring of ABCD which starts from the first character of ABCD. Note that ABCD itself is also a substring of ABCD.

In this problem, we define a special substring as a non-empty substring that contains only the same character. For example, B and CC are special substrings of ABBCCC, while ABBC and BC are not special substrings.

You are given a string S of length N and an integer K. Your task is to determine the minimum number of characters of S that need to be changed such that there exists a special substring of length K in S.

For example, let N = 6, K = 4, and S = ABBCCC. In this example, we only need to change the third character of S to C (i.e. ABBCCC → ABCCCC) so that we have a special substring CCCC of length 4.

Input begins with a line containing two integers: N K (1 ≤ K ≤ N ≤ 100 000) representing the length of the string and the length of a special substring that should be produced, respectively. The next line contains a string S containing N uppercase alphabetical character, i.e. Si ∈ [A-Z].

Output in a line an integer representing the minimum number of characters of S that need to be changed such that there exists a special substring of length K in the given S.