#include <iostream>
#include <bits/stdc++.h>
using namespace std;
#define int         long long   
#define double      long double
#define endl        "\n"
#define pb          push_back
#define PI          3.1415926535897932384626433832795l
#define F           first
#define S           second
#define mp          make_pair
#define f(i,n)      for(int i=0;i<n;i++)
#define fastio      ios::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define all(v)      (v).begin(),(v).end()
#define rall(v)     (v).rbegin(),(v).rend()
#define gcd(a,b)    __gcd((a),(b))
#define fill(a,value) memset(a,value,sizeof(a));
#define minn(v)     *min_element(v.begin(), v.end());
#define maxx(v)     *max_element(v.begin(), v.end());
#define print(x)    cout<<(x)<<endl;
#define sum(v)+x    accumulate(v.begin(), v.end(),x);
typedef pair<int,int> pii;  
typedef vector<int> vi;
/*
max and min value in a set
max_element = *(my_set.rbegin());
min_element = *my_set.begin();                                        
*/
//x.substr(start, length);
/*
Create a vector containing n 
vectors of size m.  
---vector<vector<int> > vec( n , vector<int> (m, 0));  
*/
 
/*left shifting an integer “x” with an integer “y” (x<<y) 
    is equivalent to multiplying x with 2^y (2 raise to power y).*/
/*Similarly right shifting (x>>y) is equivalent 
    to dividing x with 2^y.
*/
 
struct type{
    int k, i, j;
};
//to check if a no is prime or not?
bool isPrime(int n) 
{ 
    if (n <= 1) 
        return false; 
    if (n <= 3) 
        return true; 
    if (n % 2 == 0 || n % 3 == 0) 
        return false; 
  
    for (int i = 5; i * i <= n; i = i + 6) 
        if (n % i == 0 || n % (i + 2) == 0) 
            return false; 
  
    return true; 
}
 
// minimum subaaray sum of size k-sliding window approach
int findSubarray(int arr[], int n, int k)
{   
	int window_sum = 0;
	int min_window = INT_MAX;
	int last = 0;
	for (int i = 0; i < n; i++)
	{
		window_sum += arr[i];
		if (i + 1 >= k)
		{
			if (min_window > window_sum)
			{
				min_window = window_sum;
				last = i;
			}
			window_sum -= arr[i + 1 - k];
		}
	}
	int sum=0;
	for(int i=last-k+1;i<=last;i++)
	{
	    sum+=arr[i];
	}
	return sum;
}
//finds the next permutation to given sequence of nos
int nextpermutation(vi v)
{
    vi t=v;
    sort(all(t));
    int x=0;
    while(true)
    {
        x++;
        if(t==v)
        {
            return x;
        }
        next_permutation(all(t));
    }
}
vector<int>factors(int n) 
{ 
    // Note that this loop runs till square root 
    vi ans;
    for (int i=1; i<=sqrt(n); i++) 
    { 
        if (n%i == 0) 
        { 
            if (n == i*i )ans.pb(i);
            else
            {
                ans.pb(i);
                if (i!=n/i)ans.pb(n/i);
            }
        } 
    }
    sort(all(ans));
    return ans;
}

// Recursive C++ program to check if a string is subsequence of another string 
bool isSubSequence(char str1[], char str2[], int m, int n) 
{ 
    // Base Cases 
    if (m == 0) return true; 
    if (n == 0) return false; 
  
    // If last characters of two strings are matching 
    if (str1[m-1] == str2[n-1]) 
        return isSubSequence(str1, str2, m-1, n-1); 
  
    // If last characters are not matching 
    return isSubSequence(str1, str2, m, n-1); 
} 
void findDivisors(int n) 
{ 
  
    int div[n + 1]; 
    memset(div, 0, sizeof div); 
    for (int i = 1; i <= n; i++) { 
        for (int j = 1; j * i <= n; j++) 
            div[i * j]++; 
    } 
    int ans=0;
    for (int i = 1; i <= n; i++) 
        ans+= i*div[i];
    cout<<ans<<endl;    
}
int digits(int n)
{
    int res=0;
    while(n>0)
    {
        res++;
        n/=10;
    }
    return(res);
}
int power(int x, int y, int p) 
{ 
    int res = 1; // Initialize result 
  
    x = x % p; // Update x if it is more than or 
    // equal to p 
  
    while (y > 0) { 
        // If y is odd, multiply x with result 
        if (y & 1) 
            res = (res * x) % p; 
  
        // y must be even now 
        y = y >> 1; // y = y/2 
        x = (x * x) % p; 
    } 
    return res; 
} 
  
// Returns n^(-1) mod p 
int modInverse(int n, int p) 
{ 
    return power(n, p - 2, p); 
} 
  
// Returns nCr % p using Fermat's little 
// theorem. 
int nCr(int n,  int r, int p) 
{ 
    // Base case 
    if (r == 0) 
        return 1; 
    int fac[n + 1]; 
    fac[0] = 1; 
    for (int i = 1; i <= n; i++) 
        fac[i] = (fac[i - 1] * i) % p; 
  
    return (fac[n] * modInverse(fac[r], p) % p * modInverse(fac[n - r], p) % p) % p; 
} 
double dist (int x1, int y1, int x2, int y2) {
    double a = (x1 - x2) * (x1 - x2);
    double b = (y1 - y2) * (y1 - y2);
 
    return sqrt(a + b);
}
signed main() 
{
    fastio;
    cout << fixed << setprecision(12);
    int n;
    cin>>n;
    vi a(n),b(n);
    f(i,n)cin>>a[i]>>b[i];
    vi v;
    for(int i=0;i<n;i++)
    {
        v.pb(i);
    }
    double ans=0;
    int cnt=0;
    do { 
        for(int i=0;i<n-1;i++)
        {
            ans+=dist(a[v[i]], b[v[i]], a[v[i + 1]],  b[v[i + 1]]);
        }
        cnt++;
    } while (next_permutation(all(v))); 
    print(ans/cnt)
    
    
	return 0;
}

Measured memory_bytes: 3828