n = gets.to_i
x, y = [], []
n.times do
  xn,yn = gets.split(' ').map(&:to_i)
  x << xn
  y << yn
end
sum = 0
(0...n-1).each do |i|
  (i + 1...n).each do |j|
    _x = (x[j] - x[i])**2
    _y = (y[j] - y[i])**2
    sum += Math.sqrt(_x+_y)
  end
end
puts 2*sum/n