#include <stdio.h>

long n, sch;
char l[10001];
long c[26];

FILE *fin, *fout;

void bubblesort(void)
{
  int i, j;
  for (i=0; i<n; i++)
     {
	++c[l[i]-'a'];
        for (j=l[i]-'a'+1; j<26; j++)
          sch+=c[j];
     }
}

int main(void)
{
  fin = fopen("litere.in", "r");
  fout = fopen("litere.out", "w");
  fscanf(fin, "%ld\n", &n);
  fscanf(fin, "%s", &l);
  bubblesort();
  fprintf(fout, "%ld\n", sch);
  fclose(fout);
  return 0;
}