#include <stdio.h>

char a[10001],v[130];
long n,i,j,s;

int main(){

  FILE *f = fopen("litere.in", "r");
  FILE *g = fopen("litere.out", "w");

  fscanf(f,"%ld",&n);
  fscanf(f,"%s",a);

  for (i=0; i<n; i++) {
    v[a[i]]++;
    for (j=a[i]+1; j<='z'; j++)
      s += v[j];
  }

  fprintf(g,"%ld",s);

  fclose(f);
  fclose(g);

  return 0;
}