import std.stdio, std.algorithm, std.conv, std.string, std.array, std.math, std.range;

struct Pt {
  int x, y;
}

void main()
{
  const N = readln.chomp.to!int;
  
  auto pts = new Pt[N];
  foreach (ref pt; pts) readf(" %d %d ", &pt.x, &pt.y);
  
  const nstair = {
    long x = 1;
    foreach (i; 1..N+1) x *= i;
    return x;
  }();
  
  double avg = 0;
  foreach (perm; iota(N).permutations) {
    foreach (i; 1..N) {
      const prev = pts[perm[i-1]];
      const now  = pts[perm[i]];
      avg += sqrt((prev.x-now.x).pow(2) + (prev.y-now.y).pow(2)*1.0) / nstair;
    }
  }
  writefln("%.10f", avg);
}