#include <stdio.h>

char l[10001];
int i, n, j,a[26],m;
long contor;

FILE *f, *g;

int main()
{
  f = fopen("litere.in", "r");
  g = fopen("litere.out", "w");
  fscanf(f, "%d\n", &n);
  fscanf(f, "%s\n", &l);
  for(i=0;i<n;i++)
    {
	  m=l[i]-'a';
	  a[m]++;
	  for(j=m+1;j<26;j++)
	    contor+=a[j];
	}
  fprintf(g, "%ld\n", contor);
  fclose(g);
  return 0;
}