#include <cstdio>
#define Nmax 10001

int N,viz[27];
char S[Nmax];

void solve()
{
  long K = 0;
  for(int i=0; i<N; i++)
  {
    int k = S[i] - 'a';
    viz[k] ++;
    
    for(int j = k+1; j<27; j++)
      K += viz[j];
  }
  
  printf("%ld\n",K);
    
}

int main()
{
  freopen("litere.in","r",stdin);
  freopen("litere.out","w",stdout);
  
  scanf("%d\n%s",&N,&S);
  
  solve();  
}