N = gets.to_i
z = readlines(chomp:true).map{|line| line.split.map(&:to_i)}
num = (1..N).to_a    #必ずこれを作ってからpermutation
per = num.permutation(N).to_a     #これで各順列を全て配列perに格納した
dis = 0
for i in 0.. per.size-1
  new = per[i]
  for k in 0..N-2
    x = z[(new[k]-1)][0]-z[(new[k+1]-1)][0]
    y = z[(new[k]-1)][1]-z[(new[k+1]-1)][1]
    dis += Math.sqrt(x**2+y**2)
  end
end
puts dis.to_f/per.size