#include <bits/stdc++.h>

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

int main() {
  int n, ans = 0;
  fin >> n;
  char s[10000];
  for (int i = 0; i < n; ++i) {
    fin >> s[i];
    for (int j = 0; j < i; ++j)
      if (s[i] < s[j]) ans++;
  }
  fout << ans;
  return 0;
}