#include <fstream>

using namespace std;

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

int n;
char sir[10050];
int ap[50];
int sol;

int main() {
    fin >> n;
    fin.getline(sir, 100);
    fin.getline(sir, 10005);
    for (int i = 0; i < n; i++) {
        for (int j = sir[i] - 'a' + 1; j <= 'z' - 'a'; j++)
            sol += ap[j];
        ap[sir[i] - 'a']++;
    }
    fout << sol << '\n';
    return 0;
}