#include <fstream>

using namespace std;
char v[10010];

int f[130];
int sol, n, i, j;
int main () {
    ifstream fin ("litere.in");
    ofstream fout("litere.out");

    fin>>n;
    for (i=1;i<=n;i++)
        fin>>v[i];

    for (i=1;i<=n;i++){
        for (j=v[i]+1; j<='z'; j++)
            sol += f[j];
        f[ v[i] ]++;
    }

    fout<<sol;

    return 0;
}