#include<iostream>
#include<cstdio>
#include<cstring>
#include<cstdlib>
#include<cmath>
#include<algorithm>
using namespace std;
const int N=3e5+5;
int n,s[N];
long long ans;
void INT()
{
    int i;
    scanf("%d",&n);
    for(i=1;i<=2*n;++i) scanf("%d",&s[i]);
    sort(s+1,s+2*n+1);
}
void CHAR()
{
    int i;
    for(i=1;i<=2*n;++i) ans+=1LL*(i-1)*s[i];
    printf("%.10lf\n",1.0*ans/(2*n-1));
}
int main()
{
    INT();
    CHAR();
    return 0;
}