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

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

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