#include<bits/stdc++.h>
using namespace std;
struct time_helper{
#ifdef LOCAL
clock_t time_last;time_helper(){time_last=clock();}void test(){auto time_now=clock();std::cerr<<"time:"<<1.*(time_now-time_last)/CLOCKS_PER_SEC<<";all_time:"<<1.*time_now/CLOCKS_PER_SEC<<std::endl;time_last=time_now;}~time_helper(){test();}
#else
void test(){}
#endif
}time_helper;
#ifdef LOCAL
#include"dbg.h"
#else
#define dbg(...) (__VA_ARGS__)
#endif
namespace Fread{const int SIZE=1<<16;char buf[SIZE],*S,*T;inline char getchar(){if(S==T){T=(S=buf)+fread(buf,1,SIZE,stdin);if(S==T)return'\n';}return *S++;}}namespace Fwrite{const int SIZE=1<<16;char buf[SIZE],*S=buf,*T=buf+SIZE;inline void flush(){fwrite(buf,1,S-buf,stdout);S=buf;}inline void putchar(char c){*S++=c;if(S==T)flush();}struct NTR{~NTR(){flush();}}ztr;}
#define getchar Fread::getchar
#define putchar Fwrite::putchar
int print_precision=10;bool print_T_endl=1;char print_between=' ';
template<typename T>struct is_char{static constexpr bool value=(std::is_same<T,char>::value||std::is_same<T,signed char>::value||std::is_same<T,unsigned char>::value);};template<typename T>struct is_integral_ex{static constexpr bool value=(std::is_integral<T>::value||std::is_same<T,__int128>::value)&&!is_char<T>::value;};template<typename T>struct is_floating_point_ex{static constexpr bool value=std::is_floating_point<T>::value||std::is_same<T,__float128>::value;};namespace Fastio{struct Reader;struct Writer;template<size_t id>struct read_tuple{template<typename...T>static void read(Reader&stream,std::tuple<T...>&x){read_tuple<id-1>::read(stream,x);stream>>get<id-1>(x);}};template<>struct read_tuple<0>{template<typename...T>static void read([[maybe_unused]]Reader&stream,[[maybe_unused]]std::tuple<T...>&x){}};template<size_t id>struct print_tuple{template<typename...T>static void print(Writer&stream,const std::tuple<T...>&x){print_tuple<id-1>::print(stream,x);putchar(print_between);stream<<get<id-1>(x);}};template<>struct print_tuple<1>{template<typename...T>static void print(Writer&stream,const std::tuple<T...>&x){stream<<get<0>(x);}};template<>struct print_tuple<0>{template<typename...T>static void print([[maybe_unused]]Writer&stream,[[maybe_unused]]const std::tuple<T...>&x){}};
struct Reader{template<typename T>typename std::enable_if_t<std::is_class<T>::value,Reader&>operator>>(T&x){for(auto &y:x)*this>>y;return *this;}template<typename...T>Reader&operator>>(std::tuple<T...>&x){read_tuple<sizeof...(T)>::read(*this,x);return *this;}template<typename T>typename std::enable_if_t<is_integral_ex<T>::value,Reader&>operator>>(T&x){char c=getchar();short f=1;while(c<'0'||c>'9'){if(c=='-')f*=-1;c=getchar();}x=0;while(c>='0'&&c<='9'){x=(x<<1)+(x<<3)+(c^48);c=getchar();}x*=f;return *this;}template<typename T>typename std::enable_if_t<is_floating_point_ex<T>::value,Reader&>operator>>(T&x){char c=getchar();short f=1,s=0;x=0;T t=0;while((c<'0'||c>'9')&&c!='.'){if(c=='-')f*=-1;c=getchar();}while(c>='0'&&c<='9'&&c!='.')x=x*10+(c^48),c=getchar();if(c=='.')c=getchar();else return x*=f,*this;while(c>='0'&&c<='9')t=t*10+(c^48),s++,c=getchar();while(s--)t/=10.0;x=(x+t)*f;return*this;}template<typename T>typename std::enable_if_t<is_char<T>::value,Reader&>operator>>(T&c){c=getchar();while(c=='\n'||c==' '||c=='\r')c=getchar();return *this;}Reader&operator>>(char*str){int len=0;char c=getchar();while(c=='\n'||c==' '||c=='\r')c=getchar();while(c!='\n'&&c!=' '&&c!='\r')str[len++]=c,c=getchar();str[len]='\0';return*this;}template<typename T1,typename T2>Reader&operator>>(std::pair<T1,T2>&x){*this>>x.first>>x.second;return *this;}Reader&operator>>(std::string&str){str.clear();char c=getchar();while(c=='\n'||c==' '||c=='\r')c=getchar();while(c!='\n'&&c!=' '&&c!='\r')str.push_back(c),c=getchar();return*this;}Reader(){}}cin;const char endl='\n';
struct Writer{typedef __int128 mxdouble;template<typename T>typename std::enable_if_t<std::is_class<T>::value,Writer&>operator<<(const T&x){for(auto q:x){*this<<q;if(!is_class<decltype(q)>::value)*this<<print_between;}if(!is_class<typename T::value_type>::value&&print_T_endl)*this<<'\n';return *this;}template<typename...T>Writer&operator<<(const std::tuple<T...>&x){print_tuple<sizeof...(T)>::print(*this,x);if(print_T_endl)*this<<'\n';return *this;}template<typename T>typename std::enable_if_t<is_integral_ex<T>::value,Writer&>operator<<(T x){if(x==0)return putchar('0'),*this;if(x<0)putchar('-'),x=-x;static int sta[45];int top=0;while(x)sta[++top]=x%10,x/=10;while(top)putchar(sta[top]+'0'),--top;return*this;}template<typename T>typename std::enable_if_t<is_floating_point_ex<T>::value,Writer&>operator<<(T x){if(x<0)putchar('-'),x=-x;x+=pow(10,-print_precision)/2;mxdouble _=x;x-=(T)_;static int sta[45];int top=0;while(_)sta[++top]=_%10,_/=10;if(!top)putchar('0');while(top)putchar(sta[top]+'0'),--top;putchar('.');for(int i=0;i<print_precision;i++)x*=10;_=x;while(_)sta[++top]=_%10,_/=10;for(int i=0;i<print_precision-top;i++)putchar('0');while(top)putchar(sta[top]+'0'),--top;return*this;}template<typename T>typename std::enable_if_t<is_char<T>::value,Writer&>operator<<(const T&c){putchar(c);return*this;}Writer&operator<<(char*str){int cur=0;while(str[cur])putchar(str[cur++]);return *this;}Writer&operator<<(const char*str){int cur=0;while(str[cur])putchar(str[cur++]);return*this;}template<typename T1,typename T2>Writer&operator<<(const std::pair<T1,T2>&x){*this<<x.first<<print_between<<x.second;if(print_T_endl)*this<<'\n';return *this;}Writer&operator<<(const std::string&str){int st=0,ed=str.size();while(st<ed)putchar(str[st++]);return*this;}Writer(){}}cout;}
#define cin Fastio::cin
#define cout Fastio::cout
#define endl Fastio::endl
template<class Fun>class y_combinator_result{Fun fun_;public:template<class T>explicit y_combinator_result(T &&fun): fun_(std::forward<T>(fun)) {}template<class ...Args>decltype(auto) operator()(Args &&...args){return fun_(std::ref(*this), std::forward<Args>(args)...);}};template<class Fun>decltype(auto) y_combinator(Fun &&fun){return y_combinator_result<std::decay_t<Fun>>(std::forward<Fun>(fun));}

void init();void solve(int tc);
main()
{
	init();int t=1;
	// cin>>t;
	for(int tc=1;tc<=t;tc++)solve(tc);
}
void init()
{
}
constexpr int lim=1.25e8,nlim=1e4;
mt19937 mt(random_device{}());
void solve([[maybe_unused]]int tc)
{
	int n;
	cin>>n;
	vector<pair<int,int>>pt(n);
	cin>>pt;
	int cnt=0;
	double sum=0;
	auto sqr=[&](int v){return 1ll*v*v;};
	auto getdis=[&](int x,int y){return sqrt(sqr(pt[x].first-pt[y].first)+sqr(pt[x].second-pt[y].second));};
	if(n<=nlim)
	{
		for(int x=0;x<n;x++)
		for(int y=0;y<x;y++)
		sum+=getdis(x,y),cnt++;
	}
	else
	{
		for(int t=0;t<lim;t++)
		{
			int x=mt()%n,y=mt()%n;
			if(x==y)continue;
			sum+=getdis(x,y),cnt++;
		}
	}
	cout<<sum*n*(n-1)/2/cnt<<endl;
}