#include <bits/stdc++.h>

using namespace std;
ifstream fin("litere.in");
ofstream fout("litere.out");

int f[26];

int main() {
    int n, x, rasp = 0;
    char c;
    
    fin >> n;
    for (int i = 0; i < n; i++) {
        fin >> c;
        x = c - 97;
        for (int j = x + 1; j < 26; j++) {
            rasp += f[j];
        }
        f[x]++;
    }
    fout << rasp;
    
    return 0;
}