#include <stdio.h>
#define N 1<<14
#define omega 1<<5
int n,nr[omega],rez;
char v[N];
int main()
{
	freopen("litere.in","r",stdin);
	freopen("litere.out","w",stdout);
	scanf("%d\n",&n);
	int i,j,t;
	fgets(v+1,N,stdin);
	for (i=1; i<=n; i++)
	{
		t=(int)v[i]-96;
		for (j=t+1; j<=26; j++)
			rez+=nr[j];
		nr[t]++;
	}
	printf("%d\n",rez);
	return 0;
}