You are given a randomly generated string of character from a to z. You are required to determine the number of unique subsequence formed to this string "abcdefghijklmnopqrstuvwxyz".

Input:
First line n denoting size of string
Second line containing string s of size of n.

Example:
27
abcdefghijklmnopqrstuvwxyzz
ouput : 2
we have 2 'z' so
first unique subsequence = abcdefghijklmnopqrstuvwxyz
second unique subsequence = abcdefghijklmnopqrstuvwxyz

28
abcdefghijklmnopqrstuvwxyyzz
output: 4