#include<stdio.h>

char a[10003];
int frcv[127],j,s,n,i;

int main() {
  freopen("litere.in","r",stdin);
  freopen("litere.out","w",stdout);
  scanf("%ld",&n);
  scanf("%s",&a);
  for (i=0;i<n;i++) {
    for (j=a[i]+1;j<='z';j++)
      s+=frcv[j];
    frcv[a[i]]++;
  }
  printf("%ld\n",s);
  return 0;
}