#include <cstdio>

int f[32];
char s[16384];

int main() {
  freopen("litere.in", "r", stdin);
  freopen("litere.out", "w", stdout);

  int N;
  scanf("%d\n", &N);
  gets(s);

  ++ f[s[0] - 'a'];
  int ans = 0;
  for(int i = 1; i < N; ++ i) {
    for(char j = s[i] + 1; j <= 'z'; ++ j)
      ans += f[j - 'a'];
    ++ f[s[i] - 'a'];
  }

  printf("%d\n", ans);
  return 0;
}