def lscan; gets.split.map(&:to_i); end
def dist(u,v)
  Math::hypot(u[0]-v[0], u[1]-v[1])
end
pts = gets.to_i.times.map{ gets.split.map(&:to_f) }

sum = 0
m = 0
pts[0..-1].permutation do |ppts|
  l = ppts.shift
  t = 0
  ppts.each do |pt|
    t += dist(l, pt)
    l = pt
  end
  sum += t
  m += 1
end

p sum/m
