#include <fstream>
#include <algorithm>

using namespace std;

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

const int NMAX = 10005;
int N;
char sir[NMAX];
int v[27];


int main(){
  int nr, sol = 0;
  in >> N;
  in.get();
  in.getline(sir, NMAX - 4);
  for(int i = 0; sir[i]; ++i){
    nr = sir[i] - 'a' + 1;
    for(int j = nr + 1; j <= 26; ++j){
      sol += v[j];
    }
    v[nr]++;
  }
  out << sol << "\n";
  return 0;
}