#include<cstdio>
using namespace std;

FILE *f=fopen("litere.in","r");
FILE *g=fopen("litere.out","w");

int v[27];
int main(){
	int n,i,j,nr=0;
	char a;
	
	fscanf (f,"%d",&n);
	fscanf (f,"%c",&a);
	for(i=1;i<=n;++i){
		fscanf (f,"%c",&a);
		v[a-96]++;
		for(j=(a-95);j<=26;++j){
			if(v[j]!=0){
				nr+=v[j];
			}
		}
	}
	fprintf (g,"%d",nr);

	return 0;
}