n = gets.to_i
x = []
y = []
n.times do
  a,b = gets.split.map(&:to_i)
  x << a
  y << b
end

if n == 2
  puts Math.sqrt((((x[0]-x[1])**2)+((y[0]-y[1])**2)).to_f)
else
  z = 2.to_f/n
  sum = 0

  for i in 0..(n-1)
    for j in (i+1)..(n-1)
      root = Math.sqrt((((x[i]-x[j])**2) + ((y[i]-y[j])**2)).to_f)
      sum += root
    end
  end
  puts sum*z
end
