N = gets.chomp.to_i
a = N.times.map{gets.split.map(&:to_i)}

comb = [*(0..(N-1))].permutation.to_a
route_sum = 0
comb.each do |route|
  (0..(N-2)).each do |i|
    route_sum += ((a[route[i]][0].to_f - a[route[i+1]][0])**2 + (a[route[i]][1].to_f - a[route[i+1]][1])**2)**(0.5)
  end
end
puts route_sum / comb.length