#include <fstream>
using namespace std;
ifstream fin("litere.in");
ofstream fout("litere.out");
int n, i, j, sol, f[300];
char v[10005];
int main(){
    fin >> n;
    fin >> v + 1;
    for ( i = 1; i <= n; i++ ){
        for ( j = v[i] + 1; j <= 'z'; j++ )
            sol += f[j];
        f[ v[i] ]++;
    }
    fout << sol;
    return 0;
}