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