Problem: We have 2N pots. The market price of the i-th pot (1 ≤ i ≤ 2N) is p_i yen (the currency of Japan).

Now, you and Lunlun the dachshund will alternately take one pot. You will go first, and we will continue until all the pots are taken by you or Lunlun. Since Lunlun does not know the market prices of the pots, she will always choose a pot randomly from the remaining pots with equal probability. You know this behavior of Lunlun, and the market prices of the pots.

Let the sum of the market prices of the pots you take be S yen. Your objective is to maximize the expected value of S. Find the expected value of S when the optimal strategy is adopted.

Constraints

* 1 ≤ N ≤ 10^5
* 1 ≤ p_i ≤ 2 × 10^5
* All input values are integers.

Input

Input is given from Standard Input in the following format:


N
p_1
:
p_{2N}


Output

Print the expected value of S when the strategy to maximize the expected value of S is adopted. The output is considered correct if its absolute or relative error from the judge's output is at most 10^{-9}.

Examples

Input

1
150000
108


Output

150000.0


Input

2
50000
50000
100000
100000


Output

183333.3333333333

Solution (in CPP):

Solution:
#include<iostream>
#include<vector>
#include<algorithm>
#include<cstring>
#include<cstdio>
#include<cmath>
#include<cstdlib>
#include<ctime>
#include<queue>
#include<set>
#include<map>
#include<stack>
#include<bitset>

#define pb push_back
#define mp make_pair

using namespace std;

template<typename T>inline void upmin(T &x,T y) { y<x?x=y:0; }
template<typename T>inline void upmax(T &x,T y) { x<y?x=y:0; }

typedef unsigned int u32;
typedef long long LL;
typedef unsigned long long ULL;
typedef long double lod;
typedef pair<int,int> PR;
typedef vector<int> VI;

const lod pi=acos(-1);
const int oo=1<<30;
const LL OO=1e18;

const int N=2e5+100;

int gi() {
	int w=0;bool q=1;char c=getchar();
	while ((c<'0'||c>'9') && c!='-') c=getchar();
	if (c=='-') q=0,c=getchar();
	while (c>='0'&&c <= '9') w=w*10+c-'0',c=getchar();
	return q? w:-w;
}

/*double p[N];
int n;
inline void dfs(int s,int n,double p) {
	if (n) {
		for (int i=0;;i++)
			if (s>>i&1) {
				::p[i]+=p;
				s^=1<<i;
				break;
			}
		for (int i=0;i<::n;i++)
			if (s>>i&1)
				dfs(s^(1<<i),n-1,p/(n*2-1));
	}
	}*/
int main()
{
	/*for (n=2;n<=20;n+=2) {
		for (int i=0;i<n;i++) p[i]=0;
		dfs((1<<n)-1,n>>1,1);
		for (int i=0;i<n;i++) printf("%.5lf ",p[i]);
		putchar(10);
		}*/
	int n=gi(),i;VI w;double ans=0,t=1.0/(n*2-1);
	for (i=0;i<n*2;i++) w.pb(gi());
	sort(w.begin(),w.end());
	for (i=0;i<n*2;i++) ans+=i*t*w[i];
	printf("%.10lf\n",ans);
	return 0;
}