#include<bits/stdc++.h>
 
#define pb push_back
// #define mp make_pair
#define fi first
#define se second
#define ios ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
#define hell1 1000000007
#define hell2 1000000009
#define pi 3.1415926535
#define ms(s, n) memset(s, n, sizeof(s))
#define prec(n) fixed<<setprecision(n)
#define eps 0.000001
#define all(v) v.begin(), v.end()
#define allr(v) v.rbegin(), v.rend()
#define ll unsigned long long
#define ld long double 
#define no_of_digits(n) floor(log10(n)) + 1
#define see(x) cerr << (#x) << " is " << (x) << endl
#define INF 1e9
#define LL_MAX 1e18
// memset(array, -1, sizeof(array[0][0]) * row * count)
using namespace std;
 
ll mult(ll a,ll b, ll p=hell1){   return ((a%p)*(b%p))%p; }
ll multbig(ll a,ll b,ll mod=hell1){   if(a==0 or b==0) return 0;if(a==1 or b==1) return (a*b)%mod;ll cur=multbig(a,b/2,mod);cur=(2*cur)%mod;if(b%2) cur=(cur+a)%mod;return cur;}
ll add(ll a, ll b, ll p=hell1){ if(a>=p) a%=p; if(b>=p) b%=p; if(a+b>p) return (a + b)%p; else return a+b;}
ll sub(ll a, ll b, ll p=hell1){return (a%p - b%p + p)%p;}
ll fpow(ll n, ll k, ll p = hell1) {ll r = 1; for (; k; k >>= 1) {if (k & 1) r = mult(r,n,p); n = mult(n,n,p);} return r;}
ll inv(ll a, ll p = hell1) {return fpow(a, p - 2, p);}
ll inv_euclid(ll a, ll m = hell1){ll m0 = m;ll y = 0, x = 1;if (m == 1)return 0;while (a > 1){ll q = a / m;ll t = m;m = a % m, a = t;t = y;y = x - q * y;x = t;}if (x < 0)x += m0;return x;}
int isPowerof2(ll x) { return (x && !(x & x-1)); } 
ll log2(ll x) { ll res = 0; while (x >>= 1) res++; return res; } 
bool isPerfectSquare(long double x) {long double sr = sqrt(x); return ((sr - floor(sr)) == 0); }
bool check(ll i,ll j,ll n){return (i>=0 && i<n && j>=0 &&  j<n);} //matrix boundary checking
 
 
ll ceil(const ll &a, const ll &b){return (a - 1)/b + 1;}
 
// max({a, b, c, d})
// cout << fixed for removing e notation
 
bool comp(pair<ll, ll>a, pair<ll, ll>b)
{
    if(a.se*b.fi + b.se > b.se*a.fi + a.se) return true;
    else return false;
}
 
void convert(int arr[], int n) 
{ 
    int temp[n+1];
    for(int i=1;i<=n;i++)
        temp[i] = arr[i]; 
    // memcpy(temp, arr, n*sizeof(int)); 
  
    sort(temp+1, temp+n+1);   
    unordered_map<int, int> umap;
    int val = 1; 
    for (int i = 1; i <= n; i++) 
        umap[temp[i]] = val++;
    for (int i = 1; i <= n; i++) 
        arr[i] = umap[arr[i]]; 
}
 
bool prime[10000005];
void SieveOfEratosthenes(int n) 
{
    memset(prime, true, sizeof(prime)); 
    prime[0] = prime[1] = 0;
    for (int p=2; p*p<=n; p++) 
    {
        if (prime[p] == true) 
            for (int i=p*p; i<=n; i += p) 
                prime[i] = false; 
    }
}

int foo(int a, int b, int c)
{
    return (abs(a-b) + abs(b-c) + abs(a-c));
}


ll getFirstSetBitPos(ll n) 
{ 
    return log2(n & -n) + 1; 
} 

int main()
{
    int n;
    cin >> n;

    ll fac = 1;

    for(int i=1;i<=n;i++)
        fac *= i;

    vector<pair<int, int>>v;

    for(int i=0;i<n;i++)
    {
        int x, y;
        cin >> x >> y;

        v.pb({x, y});
    }
    sort(all(v));

    ld ans = 0.0;
    do
    {
        ld dist = 0.0;
        for(int i=1;i<v.size();i++)
        {
            // cout << "Considering points: " << v[i-1].fi << " " << v[i-1].se << " and " << v[i].fi << " " << v[i].se << "\n"; 
            ld r1 = (v[i].se - v[i-1].se)*(v[i].se - v[i-1].se);
            ld r2 = (v[i].fi - v[i-1].fi)*(v[i].fi - v[i-1].fi);

            // see(sqrt(r1+r2));

            dist += (ld)sqrt(r1+r2);
        }
        // see(dist);

        // cout << "New path\n";
        ans += dist;

    }while(next_permutation(all(v)));

    // see(dist);
    // see(fac);
    cout << prec(9);
    cout << (ld)ans/fac << "\n";
}


Measured memory_bytes: 256