#include <iostream>
#include <fstream>

using namespace std;

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

char c;
int n, sol, f[300];

int main (){
     fin>>n;
     for(int i=1; i<=n; i++){
          fin>>c;
          for(int j=c+1; j<='z'; j++)
               sol+=f[j];
          f[c]++;
     }

     fout<<sol;
     return 0;
}