#include <bits/stdc++.h>
using namespace std;
int main(){

    ifstream fin("litere.in");
    ofstream fout("litere.out");
    int n , f[123]{}, cnt = 0;
    fin >> n;

    char s[n];
    fin >> s;

    for(int i = 0; i <= n - 1; i++){
        char a = s[i];
        for(int j = 122; j > int(a); j--){
            cnt += f[j];
        }
        f[int (a)]++;
    }
    fout << cnt;
}