#include <iostream>
#include <fstream>
#include <cstring>
using namespace std;

string s;
int n, cnt = 0, f[27];

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

int main() {
    fin >> n >> s;

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

    fout << cnt;

    return 0;
}