#include<stdio.h>

char v[10000];
int a[27],s=0;
/*int cauta_minim(int aa,int n){
	int y=aa,j;
	char min=v[aa];
	for (j=aa;j<=n;++j)
		if (v[j]<min){
			min=v[j];
			y=j;
		}
	return y;
}
*/
void cauta(int i){
	int j;
	for (j=v[i]-'a'+2;j<=26;++j)
		s+=a[j];
}

int main(){
	int n,i,x,j;
	char y;
	freopen("litere.in","r",stdin);
	freopen("litere.out","w",stdout);
	scanf("%d ",&n);
	for (i=1;i<=n;++i)
		scanf("%c",&v[i]);
	for (i=1;i<=n;++i){
		/*x=cauta_minim(i,n);
		if (x>i){
			s+=x-i;
			for (j=x;j>i;--j){
				y=v[j];
				v[j]=v[j-1];
				v[j-1]=y;
			}
		}*/
		cauta(i);
		++a[v[i]-'a'+1];
	}
	printf("%d",s);
	return 0;
}