#include <fstream>
using namespace std;
ifstream fin("litere.in");
ofstream fout("litere.out");
int ap[140],i,j,sol,n;
char v[10010];
int main () {
    fin>>n;
    for (i=1;i<=n;i++) {
        fin>>v[i];
        for (j=v[i]+1;j<='z';j++) {
            sol+=ap[j];
        }
        ap[v[i]]++;
    }
    fout<<sol;
}