require 'pp'

n = gets.chomp!.to_i
pos = []
n.times do |i|
    a, b = gets.chomp!.split(" ").map(&:to_i)
    pos << [a, b]
end

len = 0
x = pos.permutation(n).to_a
x.each do |pos|
    pos.each_cons(2) do |p1, p2|
        len += Math.sqrt((p2[0] - p1[0]) ** 2 + (p2[1] - p1[1]) ** 2)
    end
end

puts len / x.length.to_f
