#include <bits/stdc++.h>

using namespace std;

ifstream fin("litere.in");
ofstream fout("litere.out");
int n, i, rasp, fr[27];
char car;

int main() {
    fin >> n;
    for(i = 1; i <= n; i++) {
        fin >> car;
        for(char j = car + 1; j <= 'z'; j++) {
            rasp += fr[j - 'a'];
        }
        fr[car - 'a']++;
    }
    fout << rasp;

    return 0;
}