#include<stdio.h>
int nr[11000],fv[30];
int main(){
	freopen("litere.in","r",stdin);
	freopen("litere.out","w",stdout);
	int n,i,sol,j;
	char aux;
	scanf("%d\n",&n);
	for(i=0;i<n;++i){
		scanf("%c",&aux);
		nr[i]=aux-'a';
	}
	sol=0;
	for(i=0;i<n;++i){
		for(j=nr[i]+1;j<26;++j)
			sol+=fv[j];
		++fv[nr[i]];
	}
	
	printf("%d\n",sol);
	
	fclose(stdin);
	fclose(stdout);
	return 0;
}