#include<stdio.h>
#include<string.h>

int frecv[125],i,nr,k;
char j;
char a[10001];

int main()
{  
    freopen("litere.in","r",stdin);
    freopen("litere.out","w",stdout);
   
   scanf("%d\n",&k);
   scanf("%s",a);
   for(i=0;i<k;i++) { for(j=97;j<=122;j++) if(frecv[j]&&a[i]<j) nr+=frecv[j];
                      frecv[a[i]]++;
                    }
  printf("%d\n",nr);
  fclose(stdin);
  fclose(stdout);
  return 0;
}