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