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

routes = [*0..n - 1].permutation
ans = 0

routes.each do |r|
  (n - 1).times do |i|
    a = r[i]
    b = r[i + 1]
    ans += Math.sqrt((xys[a][0] - xys[b][0])**2 + (xys[a][1] - xys[b][1])**2)
  end
end

puts ans / routes.count
