#require 'Math'

n=gets.strip.to_i
xy=readlines.map{|l| l.split.map(&:to_i) }

alls = (1..n).to_a.permutation

def dis(l,r)
  Math.sqrt((l[0]-r[0])**2 + (l[1]-r[1])**2)
end

result = 0
alls.each do |pa|
  pa.each_cons(2) do |ll,rr|
    l=ll-1
    r=rr-1
    result += dis(xy[l], xy[r])
  end
end

p result/alls.size.to_f
