import std.stdio;
import std.conv;
import std.string;
import std.typecons;
import std.algorithm;
import std.array;
import std.range;
import std.math;
import std.regex : regex;
import std.container;
import std.bigint;
import std.numeric;


void main()
{
  auto n = readln.chomp.to!int;
  auto p = new double[][](n, 2);
  foreach (i; 0..n) {
    p[i] = readln.chomp.split.to!(double[]);
  }

  double sum = 0.0;
  auto f = new bool[](n);
  void calc(int cnt, double dist, int prev) {
    if (cnt == n) {
      sum += dist;
      return;     
    }
    foreach (i; 0..n) {
      if (f[i]) continue;
      f[i] = true;
      auto xx = (p[prev][0] - p[i][0]) ^^ 2;
      auto yy = (p[prev][1] - p[i][1]) ^^ 2;
      auto d = sqrt(xx + yy);
      calc(cnt + 1, dist + d, i);
      f[i] = false;
    }
  }

  foreach (i; 0..n) {
    f[i] = true;
    calc(1, 0, i);
    f[i] = false;
  }

  int fact (int a) {
    if (a == 1) {
      return 1;
    }

    return a * fact(a - 1);
  }
  writefln("%.12f", sum / n / fact(n - 1));
}
