#include <bits/stdc++.h>
using namespace std;

int main() {
    int n;
    cin >> n;
    int f[27] = {0};
    long long schimbari = 0;

    for (int i = 0; i < n; i++) {
        char c;
        cin >> c;

        int x = c - 'a';

        for (int j = x + 1; j < 26; j++)
            schimbari += f[j];

        f[x]++;
    }

    cout << schimbari;
    return 0;
}