#include <iostream>
#include <fstream>

using namespace std;

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

int n, sol, j;

char c;
int f[130];

int main (){

      fin>>n;

      for(int i=1; i<=n; i++){
            fin>>c;

            for (j=c+1;j<='z';j++)
                sol += f[j];

            f[c]++;
      }

      fout<<sol;

      return 0;
}