#include <cstdio>
#include <iostream>

using namespace std;

int v[10000],lit[30];

int sum(int x)
{
	int s=0,i;
	for(i=0;i<x;++i)
		s+=lit[i];
	return s;
}

int main()
{
	freopen("litere.in","r",stdin);
	freopen("litere.out","w",stdout);
	
	int n,s=0,i;scanf("%d\n",&n);
	char c;
	
	for(i=0;i<n;++i)
	{
		scanf("%c",&c);
		s+=sum(c-'a');
		++lit[c-'a'];
	}
	printf("%d",s);
	return 0;
}