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

const string FILE_NAME = "litere";
ifstream fin(FILE_NAME + ".in");
ofstream fout(FILE_NAME + ".out");

int n, c = 0, st = 1;
int f[27];

int main() {

    string s;

    fin >> n >> s;

    for(int i = 0; i < n; i++){
        int x = int(s[i]) - 97;
        for(int j = x + 1; j < 26; j++)
            c += f[j];
        f[x]++;
    }

    fout << c;

    return 0;
}