#include <bits/stdc++.h>


#define ll long long

using namespace std;

int factorial(int n)
{
  return (n == 1 || n == 0) ? 1 : factorial(n - 1) * n;
}

int main()
{
    ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);

    int n;
    cin >> n;
    int ncopy = n;
    int a,b;
    vector< pair<int,int> > towns;
    while(n--)
    {
        cin >> a >> b;
        towns.push_back(make_pair(a,b));
    }

    long double res = 0;
    int prevA, prevB;

    sort(towns.begin(), towns.end());

    do{
        double medRes = 0;
        for(int i = 0; i < towns.size(); ++i)
        {

            if(i != 0)
            {
                double xDiff = (towns[i].first-prevA)*(towns[i].first-prevA);
                double yDiff = (towns[i].second-prevB)*(towns[i].second-prevB);
                medRes += sqrt(xDiff + yDiff);
            }
            prevA = towns[i].first;
            prevB = towns[i].second;
        }
        res += medRes;
    } while(next_permutation(towns.begin(),towns.end()));

    cout << fixed << setprecision(12) << res/(double)factorial(ncopy) << endl;






    return 0;
}
