n = gets.chomp.to_i
points = []
n.times do
  points << gets.chomp.split(' ').map(&:to_f)
end
sum = 0.0
(0...n).each do |i|
  p1 = points[i]
  (0...n).each do |j|
    next if i == j
    p2 = points[j]
    sum += (((p1[0] - p2[0]) ** 2) + ((p1[1] - p2[1]) ** 2)) ** 0.5
  end
end
puts sum / n.to_f