#include <bits/stdc++.h>
using namespace std;
template <class T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
const long long MOD = 1000000007;
const long long INF = 1e16;
const long double EPS = 1e-10;
const long double PI = 3.141592653589793;
const string abc = "abcdefghijklmnopqrstuvwxyz";
const string ABC = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
const pair<long long, long long> fd[] = {make_pair(1, 0), make_pair(-1, 0),
                                         make_pair(0, 1), make_pair(0, -1)};
long long n;
long double sum;
long double x[10], y[10];
vector<long long> v;
signed main() {
  cin >> n;
  for (long long i = (0); i < (n); i++) cin >> x[i] >> y[i];
  for (long long i = (0); i < (n); i++) v.push_back(i);
  long double cnt = 0;
  do {
    for (long long i = (0); i < (n - 1); i++) {
      sum += sqrt((x[v[i + 1]] - x[v[i]]) * (x[v[i + 1]] - x[v[i]]) +
                  (y[v[i + 1]] - y[v[i]]) * (y[v[i + 1]] - y[v[i]]));
    }
    cnt += 1.0;
  } while (next_permutation(v.begin(), v.end()));
  cout << fixed << setprecision(10) << sum / cnt << endl;
}
