counter=gets.to_i
line=[]
counter.times do |count|
  x=gets.chomp.split().map(&:to_i)
  line[count]=x
end  


x= line.permutation(counter).to_a
len=0
x.each do |p|
  p.each_cons(2) do |p1,p2|
    len += Math.sqrt((p2[0] - p1[0]) ** 2 + (p2[1] - p1[1]) ** 2)
  end  
end  

puts len / x.length.to_f