n = gets.chomp.to_i
town = Array.new(n)
n.times do |i|
  town[i] = gets.chomp.split(' ').map(&:to_i)
end

arr = [*0..n-1].permutation(n).to_a

ans = 0.0

arr.size.times do |i|
  total = 0.0
  (n-1).times do |j|
    x_delta = town[arr[i][j]][0] - town[arr[i][j+1]][0]
    y_delta = town[arr[i][j]][1] - town[arr[i][j+1]][1]
    dist = (x_delta**2 + y_delta**2) ** 0.5
    total += dist
  end
  ans += total
end
puts ans/arr.size.to_f
    