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);
  
  double sum = 0;
  int cnt;
  foreach (perm; iota(N).permutations) {
    foreach (i; 1..N) {
      const prev = pts[perm[i-1]];
      const now  = pts[perm[i]];
      sum += sqrt((prev.x-now.x).pow(2) + (prev.y-now.y).pow(2)*1f);
    }
    ++cnt;
  }
  (sum/cnt).writeln;
}