Problem:
Score : 200 points

Problem Statement

You are given a string S of length N.
Among its subsequences, count the ones such that all characters are different, modulo 10^9+7. Two subsequences are considered different if their characters come from different positions in the string, even if they are the same as strings.

Here, a subsequence of a string is a concatenation of one or more characters from the string without changing the order.

Constraints

1 \leq N \leq 100000

S consists of lowercase English letters.

|S|=N

Input

Input is given from Standard Input in the following format:

N
S

Output

Print the number of the subsequences such that all characters are different, modulo 10^9+7.

Sample Input 1

4
abcd

Sample Output 1

15

Since all characters in S itself are different, all its subsequences satisfy the condition.

Sample Input 2

3
baa

Sample Output 2

5

The answer is five: b, two occurrences of a, two occurrences of ba. Note that we do not count baa, since it contains two as.

Sample Input 3

5
abcab

Sample Output 3

17

Solution in Fortran (GNU Fortran 9.2.1) (judge verdict: Accepted)

program answer
  implicit none
  integer(16) :: i, N, ans
  integer(16) :: count(26)
  character(len=100000) :: S
  
  read(*,*) N
  read(*,*) S
  count=0
  do i = 1, N
     count(ichar(S(i:i))-96)=count(ichar(S(i:i))-96)+1
  end do

  ans=1
  do i = 1, 26
     ans=ans*(count(i)+1)
     ans=mod(ans,10**9+7)
  end do
  ans=ans-1
  write(*,*) ans
  stop
 end program answer