n=gets.to_i
xy=n.times.map{gets.split.map(&:to_i)}
 
def dis(p1, p2)
  Math.sqrt((p1[0]-p2[0])**2 + (p1[1]-p2[1])**2)
end
 
r = c = 0
[*0...n].permutation(n).each do |x|
  x.each_cons(2).each do |p1,p2|
    r += dis(xy[p1], xy[p2])
  end
  c += 1
end
puts r / c
