#pragma GCC optimize("Ofast","unroll-loops")
#pragma GCC push_options
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#include<bits/stdc++.h>
#include <xmmintrin.h>
#include <immintrin.h>
using namespace::std;
__attribute__((constructor))void init(){cin.tie(0);ios::sync_with_stdio(false);cout<<fixed<<setprecision(15);}
#include<ext/pb_ds/assoc_container.hpp>
#include<ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/priority_queue.hpp>
#include<ext/pb_ds/tag_and_trait.hpp>
// #include <boost/multiprecision/cpp_dec_float.hpp>
// #include <boost/multiprecision/cpp_int.hpp>
// namespace mp = boost::multiprecision;
// typedef mp::number<mp::cpp_dec_float<0>> cdouble;
// typedef mp::cpp_int cint;
template<typename T>using pbds=__gnu_pbds::tree<T,__gnu_pbds::null_type,less<T>,__gnu_pbds::rb_tree_tag,__gnu_pbds::tree_order_statistics_node_update>;
template<typename T>using pbds_map=__gnu_pbds::tree<T,T,less<T>,__gnu_pbds::rb_tree_tag,__gnu_pbds::tree_order_statistics_node_update>;
template<typename T,typename E>using hash_map=__gnu_pbds::gp_hash_table<T,E>;
template<typename T>using pqueue =__gnu_pbds::priority_queue<T, greater<T>,__gnu_pbds::rc_binomial_heap_tag>;
typedef long long lint;
#define INF (1LL<<60)
#define IINF (1<<30)
#define EPS (1e-10)
#define MOD 1000000007LL
//#define MOD 998244353LL
typedef vector<lint> vec;
typedef vector<vector<lint>> mat;
typedef vector<vector<vector<lint>>> mat3;
typedef vector<string> svec;
typedef vector<vector<string>> smat;
template<typename T>inline void numout(T t){bool f=0;for(auto i:t){cout<<(f?" ":"")<<i<INF/2?i:"INF";f=1;}cout<<endl;}
template<typename T>inline void numout2(T t){for(auto i:t)numout(i);}
template<typename T>inline void output(T t){bool f=0;for(auto i:t){cout<<(f?" ":"")<<i;f=1;}cout<<endl;}
template<typename T>inline void output2(T t){for(auto i:t)output(i);}
template<typename T>inline void _output(T t){bool f=0;for(lint i=0;i<t.size();i++){cout<<f?"":" "<<t[i];f=1;}cout<<endl;}
template<typename T>inline void _output2(T t){for(lint i=0;i<t.size();i++)output(t[i]);}
#define rep(i,n) for(lint i=0;i<lint(n);++i)
#define repi(i,a,b) for(lint i=lint(a);i<(lint)(b);++i)
#define rrep(i,n) for(lint i=lint(n)-1;i>=0;--i)
#define rrepi(i,a,b) for(lint i=lint(b)-1;i>=lint(a);--i)
#define irep(i) for(lint i=0;;++i)
#define all(n) begin(n),end(n)
#define dist(a,b,c,d) sqrt(pow(a-c,2)+pow(b-d,2))
inline lint gcd(lint A,lint B){return B?gcd(B,A%B):A;}
inline lint lcm(lint A,lint B){return A/gcd(A,B)*B;}
// inline cint cgcd(cint A,cint B){return B?cgcd(B,A%B):A;}
// inline cint clcm(cint A,cint B){return A/cgcd(A,B)*B;}
inline bool chmin(auto& s,const auto& t){bool res=s>t;s=min(s,t);return res;}
inline bool chmax(auto& s,const auto& t){bool res=s<t;s=max(s,t);return res;}
const vector<lint> dx={1,0,-1,0,1,1,-1,-1};
const vector<lint> dy={0,1,0,-1,1,-1,1,-1};
#define SUM(v) accumulate(all(v),0LL)
auto call=[&](auto f,auto... args){return f(f,args...);};

template<typename T>
class AVL_set{
	struct node;
	using np=node*;
    struct node{
        T val;
        int cnt=1;
        int dep=0;
        np ch[2]={nullptr,nullptr};
        node(T val):val(val){}
    };
    np root=nullptr;
    inline int count(np t){return t?t->cnt:0;}
    inline int depth(np t){return t?t->dep:0;}
    inline np update(np& t,bool f=1){
		if(!t)return t;
        t->cnt=count(t->ch[0])+1+count(t->ch[1]);
        t->dep=max(depth(t->ch[0]),depth(t->ch[1]))+1;
        if(f&&abs(depth(t->ch[0])-depth(t->ch[1]))==2){
			rebuild(t,depth(t->ch[0])<depth(t->ch[1]));
		}
        return t;
    }
    public:
	AVL_set(){}
    inline void insert(T val){insert(val,root);}
    inline void erase(T val){erase(val,root);}
    inline int lower_bound(T val){return lower_bound(val,root);}
	inline int upper_bound(T val){return upper_bound(val,root);}
    inline int count(T val){return upper_bound(val,root)-lower_bound(val,root);}
	inline int size(){return count(root);}
	inline T find_by_order(int idx){assert(0<=idx&&idx<size());return find_by_order(idx,root);}
	inline T operator[](int idx){return find_by_order(idx);}
    private:
    void insert(T val,np& t){
        if(!t)t=new node(val);
        else if(val<=t->val)insert(val,t->ch[0]);
        else if(val>t->val)insert(val,t->ch[1]);
        update(t);
    }
    void erase(T val,np& t){
        if(!t)return;
        else if(val==t->val){
			if(!t->ch[0]||!t->ch[1])t=t->ch[0]?t->ch[0]:t->ch[1];
			else move_down(t->ch[0],t);
		}
        else if(val<t->val)erase(val,t->ch[0]);
        else if(val>t->val)erase(val,t->ch[1]);
		update(t);
    }
	void move_down(np& t,np& p){
		if(t->ch[1])move_down(t->ch[1],p);
		else p->val=t->val,t=t->ch[0];
		update(t);
	}
    int lower_bound(T val,np t){
        if(!t)return 0;
        if(val<=t->val)return lower_bound(val,t->ch[0]);
        else return count(t->ch[0])+1+lower_bound(val,t->ch[1]);
    }
	int upper_bound(T val,np t){
        if(!t)return 0;
        if(val<t->val)return upper_bound(val,t->ch[0]);
        else return count(t->ch[0])+1+upper_bound(val,t->ch[1]);
    }
	T find_by_order(int idx,np t){
		if(idx==count(t->ch[0]))return t->val;
		else if(idx<count(t->ch[0]))return find_by_order(idx,t->ch[0]);
		else return find_by_order(idx-count(t->ch[0])-1,t->ch[1]);
	}
	np rot(np t){
		int b=depth(t->ch[0])<depth(t->ch[1]);
		if(depth(t->ch[0])==depth(t->ch[1]))return t;
		np s=t->ch[b];
		t->ch[b]=s->ch[1-b];
		s->ch[1-b]=t;
		update(t,0);update(s,0);
		return s;
	}
    void rebuild(np& t,int b){
		if(t->ch[b]&&depth(t->ch[b]->ch[b])<depth(t->ch[b]->ch[1-b])){
			t->ch[b]=rot(t->ch[b]);
		}
		t=rot(update(t,0));
    }
};

int main(){
	lint n;
	cin>>n;
	vec x(n),y(n);
	rep(i,n){
		cin>>x[i]>>y[i];
	}
	double ans=0;
	rep(i,n)rep(j,i){
		ans+=dist(x[i],y[i],x[j],y[j]);
	}
	ans*=2./n;
	cout<<ans<<endl;
}


Measured memory_bytes: 256