#include <bits/stdc++.h>
using namespace std;
int main() {
    ifstream fin("litere.in");
    ofstream fout("litere.out");
    int n, f[26] = {0};
    long long sol = 0;
    fin >> n;
    for (int i = 1; i <= n; i++) {
        char c;
        fin >> c;
        int poz = c - 'a';
        for (int j = poz + 1; j < 26; j++)
            sol += f[j];
        f[poz]++;
    }
    fout << sol;
    return 0;
}