#include <stdio.h>
int n, v[30], contor, c;
char s[10004];

int main()
{
   freopen("litere.in","r",stdin);
   freopen("litere.out","w",stdout);
   int i,j;
   scanf("%d",&n);
   scanf("%s",&s);
   for (i = 1; i < 27; i++)
   {
      c = 0;
      for (j = 0; j < n; j++)
	 if (s[j] == 'a' + i - 1) contor += c;
	 else if (s[j] > 'a' + i - 1) c++;
   }
   printf("%d\n",contor);
   return 0;
}