#include <fstream>
using namespace std;

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

int n, ans, fr[30];
string s;

int main() {
    fin >> n;
    fin >> s;
    fin.close();
    for(int i = 0; i < n; i++) {
        fr[s[i]]++;
        for(int j = s[i] + 1; j <= 'z'; j++) {
            ans += fr[j];
        }
    }
    fout << ans;
    return 0;
}