n = gets.to_i
ps = []
n.times do
  x,y = gets.chomp.split.map(&:to_i)
  ps << [x,y]
end

result = 0.0;
(0...n).to_a.permutation.each do |per|
  per.each_cons(2) do |a,b|
    aa = ps[a]
    bb = ps[b]
    result += Math::sqrt((aa[0] - bb[0])**2 + (aa[1] - bb[1])**2)
  end
end

result /= ((1..n).inject(&:*))

puts result
