#include<bits/stdc++.h>
//#include <boost/multiprecision/cpp_int.hpp>
//using namespace boost::multiprecision;
 
#define ll int
#define ld long double
#define bigint int64_t
#define vll vector<ll>
#define vpll vector<pair<ll,ll>>
#define mp make_pair
#define pb push_back
#define MOD 1000000007
#define flash ios_base::sync_with_stdio(false);cin.tie(NULL);
#define test ll t;read(t);while(t--)
#define all(v) v.begin(),v.end()
#define pll pair<ll,ll>
 
/*#include <ext/pb_ds/assoc_container.hpp> // Policy_based_Data_Structure 
#include <ext/pb_ds/graph_policy.hpp> 
using namespace __gnu_pbds;*/ 
 
#define forn(i,n) for(ll (i) = 0 ; (i) < (n) ; ++(i))
#define for1(i,n) for(ll (i) = 1 ; (i) <= (n) ; ++(i))
#define forr(i,n) for(ll (i) = (n)-1 ; (i)>=0 ; --(i))
#define forab(i,a,b,c) for(ll (i) = a ; (i) <= (b) ; (i)+=(c))
 
using namespace std;
template <typename T>
void read(T &x){
    cin >> x;
}
template <typename T , typename T0>
void read(T &x,T0 &y){
    cin >> x >> y;
}
 
template <typename T , typename T0 , typename T1>
void read(T &x,T0 &y,T1 &z){
    cin >> x >> y >> z;
}
 
template <typename T , typename T0 , typename T1 , typename T2>
void read(T &x,T0 &y,T1 &z,T2 &w){
    cin >> x >> y >> z >> w;
}
 
template <typename T , typename T0>
void read(pair< T , T0 > &p){
    cin >> p.fst >>p.scd;
}
 
template <typename T>
void read(vector< T > &oneD,ll n){
    forn(i,n)
    {
        ll x;
        read(x);
        oneD.push_back(x);
    }
}
 
template <typename T>
void read(T oneD[] , ll n){
    for(ll i=0;i<n;i++){
        read(oneD[i]);
    }
}
 
template <typename T>
void write(T &x){
    
    cout << x << " ";
}
 
template <typename T , typename T0>
void write(T &x,T0 &y){
    cout << x << " " << y << "\n";
}
 
template <typename T , typename T0 , typename T1>
void write(T &x,T0 &y,T1 &z){
    cout << x << " " << y << " " << z << "\n";
}
 
template <typename T , typename T0 , typename T1 , typename T2>
void write(T &x,T0 &y,T1 &z,T2 &w){
    cout << x << " " << y << " " << z << " " << w << "\n";
}
 
template <typename T , typename T0>
void write(pair< T , T0 > &p){
    write(p.fst);
    write(p.scd);
    cout << endl;
}
 
template <typename T>
void write(vector< T > &oneD,ll n){
    for(ll i=0;i<n;i++){
        cout<<oneD[i]<<" ";
    }
    cout << endl;
}
 
template <typename T>
void write(T oneD[] ,int n){
    for(ll i=0;i<n;i++){
        write(oneD[i]);
    }
    cout << endl;
}
 
template <typename T , typename T0>
void write(map< T , T0 > &mpp){
    for(auto it : mpp){
        write(it.fst);
        cout << ": ";
        write(it.scd);
        cout << "\n";
    }
    cout<<endl;
}
 
vll seive;
void Seive(){
    const ll maxn = 1000005;
    seive.resize(maxn);
    forn(i,maxn) seive[i] = i;
    forab(i,2,maxn,2)
    seive[i]=2;
    seive[1] = -1;
    seive[0] = -1;
    for(ll i=3;i<=maxn;i+=2) if(i == seive[i]) for(ll j = i*i ; j < maxn ; j+=i) if(seive[j] == j) seive[j] = i;
}
 
ll extended_GCD(ll a , ll b , ll &x , ll &y){
    if(a == 0){
        x = 0;
        y = 1;
        return b;
    }
    ll x1 , y1;
    ll gcd = extended_GCD(b%a , a , x1 , y1);
    x = y1 - (b/a)*x1; 
    y = x1;
    return gcd;
}
 
ll modinv(ll a , ll mod = MOD){
    ll x , y;
    extended_GCD(a , mod , x , y);
    if(x < 0) x += mod;
    return x;
}
 
//Chinese Remainder Theoram
ll find_x_CRT(ll num[],ll rem[],ll n)
{
    ll pro=1;
    forn(i,n)
    pro*=num[i];
    ll res=0;
    forn(i,n)
    {
        ll pp = pro/num[i];
        res+=(rem[i]*modinv(pp,num[i])*pp);
    }
    res%=pro;
    return res;
}
 
ll power(ll a, ll b, ll m = MOD) {
    a%=m;
    ll res = 1;
    while (b > 0) {
        if (b & 1)
            res = res * a %m;
        a = a * a %m;
        b >>= 1;
    }
    return res;
}
 
ll powe(ll a, ll b, ll m = MOD) {
    ll res = 1;
    while (b > 0) {
        if (b & 1)
            res = res * a;
        a = a * a;
        b >>= 1;
    }
    return res;
}
 
ll gcd(ll a, ll b)
{
    if (a == 0)
        return b;
    return gcd(b % a, a);
}
 
string getString(char x) 
{ 
    string s(1, x); 
    return s;    
} 
 
//typedef graph<ll, null_type, less<ll>, rb_graph_tag,graph_order_bigsmallistics_node_update> index_set
void add_edge(ll u,ll v,vector<vll> &ad)
{
    ad[u].pb(v);
    ad[v].pb(u);
}

void update(ll index,ll st,ll en,ll qs,ll qe,ll inc, vll &tree,vll &lazy)
{
    if(lazy[index]!=0)
    {
        tree[index]+=((en-st+1)*lazy[index]);
        if(st!=en)
        {
            lazy[2*index]+=lazy[index];
            lazy[2*index+1]+=lazy[index];
        }
        lazy[index]=0;
    }
    if(qs>en||qe<st)
    return;
    if(st>=qs&&en<=qe)
    {
        if(st!=en)
        {
            lazy[2*index]+=inc;
            lazy[2*index+1]+=inc;
        }
        tree[index]+=(en-st+1)*inc;
        return;
    }
    ll mid = (st+en)/2;
    update(2*index,st,mid,qs,qe,inc,tree,lazy);
    update(2*index+1,(mid+1),en,qs,qe,inc,tree,lazy);
    tree[index]=tree[2*index]+tree[2*index+1];
}

ll query(ll index,ll st,ll en,ll qs,ll qe,vll &tree,vll &lazy)
{
    if(lazy[index]!=0)
    {
        tree[index]+=(en-st+1)*lazy[index];
        if(st!=en)
        {
        lazy[2*index]+=lazy[index];
        lazy[2*index+1]+=lazy[index];
        }
        lazy[index]=0;
    }
    if(qs>en||qe<st)
    return 0;
    if(st>=qs&&en<=qe)
    {
        return tree[index];
    }
    ll mid = (st+en)/2;
    ll left = query(2*index,st,mid,qs,qe,tree,lazy);
    ll right = query(2*index+1,(mid+1),en,qs,qe,tree,lazy);
    return (left+right);
}

ld factorial(ll x)
{
    ld fact = 1.0;
    forab(i,1,x,1)
    {
        fact*=i;
    }
    return fact;
}

ld distan(ll x1,ll y1,ll x2,ll y2)
{
    ld ans = abs(x1-x2)*abs(x1-x2)+abs(y1-y2)*abs(y1-y2);
    ans = sqrt(ans);
    return ans;
}

int main()
{
    flash;
    ll n;
    read(n);
    ll a[n];
    forn(i,n)
    a[i]=i;
    map<ll,pll> m;
    forn(i,n)
    {
        ll x,y;
        read(x,y);
        m[i]={x,y};
    }
    ld total = factorial(n);
    ld dis = 0.0;
    do
    {
        forn(i,(n-1))
        {
            dis+=distan(m[a[i]].first,m[a[i]].second,m[a[i+1]].first,m[a[i+1]].second);
        }
    } while(next_permutation(a, a + n));
    ld f = dis/total;
    cout<<fixed<<setprecision(10)<<f;
    return 0;
}

Measured memory_bytes: 256