#include <fstream>
using namespace std;

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

int main() {
    int n, nrsc = 0;
    int v[27] = {0};

    fin >> n;
    for (int i = 0; i < n; i++) {
        char x;
        int c;
        fin >> x;
        c = x - 'a';
        for (int j = c + 1; j <= 26; j++) {
            nrsc += v[j];
        }
        v[c]++;
    }

    fout << nrsc;

    return 0;
}