#include<fstream>
using namespace std;
int n, i, j, sum, suma;
int v[150];
char s[10005];
ifstream fin("litere.in");
ofstream fout("litere.out");
int main(){
    fin>> n;
    fin>> s;
    sum = 0;
    for(i = 0; i < n; i++){
        suma = 0;
        for(j = s[i] + 1; j <= 'z'; j++){
            suma += v[j];
        }
        sum+= suma;
        v[s[i]]++;
    }
    fout<< sum;
    return 0;
}