#include <fstream>

using namespace std;

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

const int NMAX = 10005;
char sir[NMAX];
int V[30];
int L, sol;


int main(){
    in >> L; in.get();
    in >> sir;
    for(int i = 0; sir[i] ; ++i){
        int index = sir[i] - 'a' + 1;
        for(int j = index + 1; j <= 26; ++j)
            sol += V[j];
        V[index]++;
    }
    out << sol << "\n";    
    return 0;
}