 N = gets.to_i
 inputs = []
 N.times do |i|
   x,y = gets.split(" ")
   inputs.push({x: x.to_f, y: y.to_f})
end

permu = inputs.permutation(N).to_a

res = []
permu.each do |a|
  sum = 0

  (a.length - 1).times do |i|
    sum += Math.hypot(a[i][:x] - a[i+1][:x], a[i][:y] - a[i+1][:y])
  end

  res.push(sum)
end

ave = (res.inject(:+) / res.length).round(10)

puts ave