#include<cstdio>
#include<algorithm>
using namespace std;
long long n,s[200005];
long long ans;
int main(){
	scanf("%lld",&n);
	for (int i=0;i<n*2;i++)scanf("%lld",&s[i]);
	sort(s,s+n*2);
	for (long long i=0;i<n*2;i++)ans+=i*s[i];
	printf("%.10lf",ans/(double)(2*n-1));
	return 0;
}