n=gets.to_i
x,y=n.times.map{gets.split.map(&:to_f)}.transpose

count=0
count_n=0
[*1..n].permutation.to_a.each do |perm|
  (n-1).times do |i|
    count+=((x[perm[i+1]-1]-x[perm[i]-1])**2+(y[perm[i+1]-1]-y[perm[i]-1])**2)**0.5
  end
  count_n+=1
end
puts sprintf("%.7f",count/count_n)
