N = gets.to_i
X = 0
Y = 1
CITIES = N.times.map { gets.split.map!(&:to_i) }

dist = 0
CITIES.permutation(N) do |cities|
  cities.each_cons(2) do |a, b|
    dist += Math.sqrt((a[X] - b[X])**2 + (a[Y] - b[Y])**2)
  end
end

puts dist / 2.upto(N).inject(:*)