#include <stdio.h>
int n, v[30], contor;
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 = 0; i < n; i++)
   {
      for (j = 30; j >= s[i] - 'a'; j--) v[j]++;
      contor += v[s[i]-'a']-1;
   }
   printf("%d\n",contor);
   return 0;
}