#include <bits/stdc++.h>
                                                    #include<iostream>
                                                    #include<cstdio>
                                                    #include<vector>
                                                    #include<queue>
                                                    #include<map>
                                                    #include<cstring>
                                                    #include<string>
                                                    #include <math.h>
                                                    #include<algorithm>
                                                //    #include <boost/multiprecision/cpp_int.hpp>
                                                    #include<functional>
                                           #define int long long
                                                    #define inf  10000000000007
                                                    #define pa pair<int,int>
                                                    #define ll long long
                                                    #define pal pair<double,pa>
                                                    #define ppa pair<pa,int>
                                                    #define ppap pair<int,pa>
                                                    #define ssa pair<string,int>
                                                    #define  mp make_pair
                                                    #define  pb push_back
                                                    #define EPS (1e-10)
                                                    #define equals(a,b) (fabs((a)-(b))<EPS)
                                             
                                                    using namespace std;
                                          
                                                    class Point{
                                                    	public:
                                                    	double x,y;
                                                    	Point(double x=0,double y=0):x(x),y(y) {}
                                                    	Point operator + (Point p) {return Point(x+p.x,y+p.y);}
                                                    	Point operator - (Point p) {return Point(x-p.x,y-p.y);}
                                                    	Point operator * (double a) {return Point(x*a,y*a);}
                                                    	Point operator / (double a) {return Point(x/a,y/a);}
                                                    	double absv() {return sqrt(norm());}
                                                    	double norm() {return x*x+y*y;}
                                                    	bool operator < (const Point &p) const{
                                                    		return x != p.x ? x<p.x: y<p.y;
                                                    	}
                                                    	bool operator == (const Point &p) const{
                                                    		return fabs(x-p.x)<EPS && fabs(y-p.y)<EPS;
                                                    	}
                                                    };
                                                    typedef Point Vector;
                                             
                                                    struct Segment{
                                                    Point p1,p2;
                                                    };
                                                     double dot(Vector a,Vector b){
                                                    	return a.x*b.x+a.y*b.y;
                                                    }
                                                    double cross(Vector a,Vector b){
                                                    	return a.x*b.y-a.y*b.x;
                                                    }
                                                
                        bool parareru(Point a,Point b,Point c,Point d){
                        //	if(abs(cross(a-b,d-c))<EPS)cout<<"dd "<<cross(a-b,d-c)<<endl;
                        	return abs(cross(a-b,d-c))<EPS;
                        }
                        double distance_ls_p(Point a, Point b, Point c) {
                          if ( dot(b-a, c-a) < EPS ) return (c-a).absv();
                          if ( dot(a-b, c-b) < EPS ) return (c-b).absv();
                          return abs(cross(b-a, c-a)) / (b-a).absv();
                        }
                        bool is_intersected_ls(Segment a,Segment b) {
                        	if(a.p1==b.p1||a.p2==b.p1||a.p1==b.p2||a.p2==b.p2) return false;
                        	if(parareru((a.p2),(a.p1),(a.p1),(b.p2))&&parareru((a.p2),(a.p1),(a.p1),(b.p1))){
                        //		cout<<"sss"<<endl;
                        		if(dot(a.p1-b.p1,a.p1-b.p2)<EPS) return true;
                        		if(dot(a.p2-b.p1,a.p2-b.p2)<EPS) return true;
                        		if(dot(a.p1-b.p1,a.p2-b.p1)<EPS) return true;
                        		if(dot(a.p1-b.p2,a.p2-b.p2)<EPS) return true;
                        		return false;
                        	}
                          else return ( cross(a.p2-a.p1, b.p1-a.p1) * cross(a.p2-a.p1, b.p2-a.p1) < EPS ) && ( cross(b.p2-b.p1, a.p1-b.p1) * cross(b.p2-b.p1, a.p2-b.p1) < EPS );
                        }
                         
                        double segment_dis(Segment a,Segment b){
                        	if(is_intersected_ls(a,b))return 0;
                        	double r=distance_ls_p(a.p1, a.p2, b.p1);
                        	r=min(r,distance_ls_p(a.p1, a.p2, b.p2));
                        	r=min(r,distance_ls_p(b.p1, b.p2, a.p2));
                        	r=min(r,distance_ls_p(b.p1, b.p2, a.p1));
                        	return r;
                        }
                        Point intersection_ls(Segment a, Segment b) {
                          Point ba = b.p2-b.p1;
                          double d1 = abs(cross(ba, a.p1-b.p1));
                          double d2 = abs(cross(ba, a.p2-b.p1));
                          double t = d1 / (d1 + d2);
                         
                          return a.p1 + (a.p2-a.p1) * t;
                        }
                         
                                        string itos( int i ) {
                                        ostringstream s ;
                                        s << i ;
                                        return s.str() ;
                                        }
                                         
                                        int gcd(int v,int b){
                                        	if(v>b) return gcd(b,v);
                                        	if(v==b) return b;
                                        	if(b%v==0) return v;
                                        	return gcd(v,b%v);
                                        }
                         
                                        double distans(double x1,double y1,double x2,double y2){
                                        	double rr=(x1-x2)*(x1-x2)+(y1-y2)*(y1-y2);
                                        	return sqrt(rr);
                                        	
                                        }
                                        /*
                        int pr[100010];
                        //int inv[100010];
                        */ 
                        int beki(int wa,int rr,int warukazu){
                        	if(rr==0) return 1ll;
                        	if(rr==1) return wa%warukazu;
                        	if(rr%2==1) return (beki(wa,rr-1,warukazu)*wa)%warukazu;
                        	int zx=beki(wa,rr/2,warukazu);
                        	return (zx*zx)%warukazu;
                        }
                          /*
                        void gya(){
                        	pr[0]=1;
                        	for(int i=1;i<100010;i++){
                        		pr[i]=(pr[i-1]*i)%inf;
                        	}
                        	for(int i=0;i<100010;i++) inv[i]=beki(pr[i],inf-2);
                        	
                        }
                         
                        */
                         //sort(ve.begin(),ve.end(),greater<int>());
                                                 //----------------kokomade tenpure------------
                        //vector<double> ans(100000000),ans2(100000000);
                        
                        int par[200100],ranks[200100],kosuu[200100];     
                         
                        void shoki(int n){
                        	for(int i=0;i<n;i++){
                        		par[i]=i;
                        		ranks[i]=0;
                        		kosuu[i]=1;
                        	}
                        }
                         
                        int root(int x){
                        	return par[x]==x ? x : par[x]=root(par[x]);
                        }
                         
                        bool same(int x,int y){
                        return root(x)==root(y);
                        }
                         
                        void unite(int x,int y){
                         x=root(x);
                         y=root(y);
                        	int xx=kosuu[x],yy=kosuu[y];
                         if(x==y) return;
                        	if(ranks[x]<ranks[y]){
                        		par[x]=y;
                        		kosuu[y]=yy+xx;
                        	}
                         else {
                        	par[y]=x;
                        	if(ranks[x]==ranks[y]) ranks[x]=ranks[x]+1;
                         	kosuu[x]=yy+xx;
                         }
                        	return;
                        }
                        
            int N=262144;
            pa sgt[530000];
             
            pa minis(int i,int j ){
            if(i==j) return sgt[i];
            	else if(i%2==1){
            		pa z=minis(i+1,j);
            		if(sgt[i].first>z.first)return sgt[i] ;
            		else return z;
            	}
            		else if(j%2==0) 	{
            		pa z=minis(i,j-1);
            		if(sgt[j].first>z.first)return sgt[j] ;
            		else return z;
            	}
            else return minis(i/2,j/2);
            }
             
            pa sgtmin(int i ,int j) {
            	return minis(i+N,j+N);
            }
             
            void upd(int x,int y){
            	x+=N;
            sgt[x].first=y;
            	sgt[x].second=x;
            x=x/2;
            while(x != 0){
            	if(sgt[2*x+1].first>sgt[2*x].first){
            		sgt[x]=sgt[2*x+1];
            	}
            	else sgt[x]=sgt[2*x];
             
            x=x/2;
            }
            }
              
             
                 int n,p;
            int x[100030],sc[100030]={0};
            int rui[100040]={0};
            int score[100040];
                 vector<int> ve;
                    signed main(){
            int n;
                    	cin>>n;
                    	for(int i=1;i<=2*n;i++){
        int y;
        cin>>y;
        ve.pb(-y);
        }
        sort(ve.begin(),ve.end());
        for(int i=0;i<2*n;i++) ve[i] *=-1;
         
        double ans=0;
            for(int i=0;i<2*n;i++){
        ans+= (ve[i]+0.0)*(2*n-(i+1)+0.0)/(2*n-1.0);
        }
                    	printf("%.10lf\n",ans);
                    	return 0;
                    }

Measured memory_bytes: 2420