n = gets.to_i
x,y = [],[]
n.times do
  xn,yn = gets.split(' ').map(&:to_i)
  x << xn
  y << yn
end
fact = (1..n).inject(:*)
pem = (0...n).to_a.permutation(n).to_a
s = []
sum = 0
pem.each do |ar|
  (1...n).each do |i|
    _x = x[ar[i]] -x[ar[i-1]]
    _y = y[ar[i]] -y[ar[i-1]]
    sum += Math.sqrt(_x**2+_y**2)
  end
end

puts sum/fact