#include <fstream>

#define ASCII_DIM 130
#define DIM 10001

using namespace std;

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

int n, sol;
char s[DIM];
int f[ASCII_DIM];

int main() {
    fin >> n >> s;
    for (int i = 0; i < n; i++) {
        f[s[i]]++;
        for (int j = s[i] + 1; j <= 'z'; j++)
            sol += f[j];
    }

    fout << sol;

    return 0;
}