#include <iostream>
#include <fstream>

using namespace std;

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

#define ll long long
int const nmax = 10000;
char v[5 + nmax];
int frec[5 + nmax];

int main()
{
  int n;
  in >> n;
  ll result = 0;
  for(int i = 1; i <= n;i++) {
    in >> v[i];
    int val = v[i] - 'a';
    for(int j = val + 1; j < 26; j++)
      result += frec[j];
    frec[val]++;
  }
  out << result;

  return 0;
}