#include <cstdio>
using namespace std;
int v[27],n,i,nr,c,j;
char a[10000];
int main() {
	freopen("litere.in","r",stdin);
	freopen("litere.out","w",stdout);
	scanf("%d",&n);
	scanf("%s",a);
	for (i=0;i<n;i++) {
		c=(int)a[i]-96;
		for (j=c+1;j<=26;j++) nr+=v[j];
		v[c]++;
	}
	printf("%d",nr);
	return 0;
}