n = gets.to_i
p = Array.new(n) { gets.chomp.split.map(&:to_f) }
d = 0.0
0.step(n - 2) do |i|
  (i + 1).step(n - 1) do |j|
    xi, yi = p[i]
    xj, yj = p[j]
    d += Math::sqrt((xj - xi) ** 2 + (yj - yi) ** 2)
  end
end
puts "%.7f" % [d * 2.0 / n]