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 C++14 (GCC 5.4.1) (judge verdict: Accepted)

#define _USE_NATH_DEFINES
#include <cstdio>
#include <iostream>
#include <sstream>
#include <fstream>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <complex>
#include <string>
#include <vector>
#include <list>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <bitset>
#include <numeric>
#include <limits>
#include <climits>
#include <cfloat>
#include <functional>
#include <iterator>
#include <random>
#include <tuple>


using namespace std;
#define REP(i,m,n) for(int i=(int)m ; i < (int) n ; ++i )
#define rep(i,n) REP(i,0,n)
using ll = long long;
#define EPS 1e-9
const int INF = 1050000000;
const long long LINF = 1LL << 60;
const ll MOD = 1e9 + 7;
const int MINF = -1050000000;



int main() {
	int n;
	string s;
	cin >> n >> s;

	string abc = "abcdefghijklmnopqrstuvwxyz";
	ll ans = 1;
	ll cnt = 1;
	rep(i, abc.size()) {
		rep(j, s.size()) {
			if (abc[i] == s[j]) cnt++;
		}
		ans *= cnt;
		ans = ans % MOD;
		cnt = 1;
	}
	ans--;
	cout << ans << endl;
	return 0;
}