# frozen_string_literal: true

n = gets.to_i
xy = n.times.map { gets.split.map(&:to_i) }

def dist(x0, y0, x1, y1)
  Math.sqrt((x0 - x1)**2 + (y0 - y1)**2)
end

total = 0
p = (0..(n - 1)).to_a.permutation(n)
p.each do |r|
  sum = 0
  1.upto(n - 1) do |i|
    sum += dist(xy[r[i]][0], xy[r[i]][1], xy[r[i - 1]][0], xy[r[i - 1]][1])
  end
  total += sum
end
puts total / p.size
