#include<cstdio>
using namespace std;
int n,i,nr[30],p[10003],k,j;
char s[10003];
int main()
{
	freopen("litere.in","r",stdin);
	freopen("litere.out","w",stdout);
	scanf("%d",&n);
	scanf("%s",s);
	for(i=0;i<n;i++)
	{
		k=0;
		for(j=s[i]+1;j<='z';j++)
			k+=nr[j-'a'];
		p[i]=p[i-1]+k;
		nr[s[i]-'a']++;
	}
	printf("%d",p[n-1]);
	return 0;
}