n = gets.chomp.to_i
xy = Array.new(n)
n.times do |i|
	xy[i] = gets.chomp.split.map(&:to_i)
end
aves = []
(0..n-1).to_a.permutation.each do |perm|
	ave = 0
	(n-1).times do |i|
		ave += Math.sqrt((xy[perm[i+1]][0] - xy[perm[i]][0]) ** 2 + (xy[perm[i+1]][1] - xy[perm[i]][1]) ** 2)
	end
	aves << ave 
end

puts aves.inject(:+) / aves.length

