n = gets.to_i
cs = n.times.map{gets.chomp.split.map(&:to_i)}
sum = 0
[*0..n-1].permutation.each do |a|
  a.each_cons(2) do |u,v|
    i,j = cs[u]
    x,y = cs[v]
    sum += Math.sqrt((x - i)**2 + (y - j)**2)
  end
end
puts sum / (1..n).inject(:*)