#include <bits/stdc++.h>

using namespace std;

ifstream fin("litere.in");
ofstream fout("litere.out");
int n, i, r, fr[32];
char c[10002];

int main() {
    fin >> n >> (c + 1);

    for(i = 1; i <= n; i++) {
        int car = c[i] - 'a';
        for(int j = car + 1; j < 26; j++) r += fr[j];
        fr[car]++;
    }
    fout << r;

    return 0;
}