#include<stdio.h>
int nr,n,V[30],i,j;
char a[10010];
int main(){
	freopen("litere.in","r",stdin);
	freopen("litere.out","w",stdout);
	scanf("%d\n",&n);
	scanf("%s",&a);
	for(i=0;i<=n-1;i++){
		for(j=a[i]-'a'+1;j<='z'-'a';j++)
			nr+=V[j];
		V[a[i]-'a']++;
	}
	printf("%d",nr);
	return 0;
}