n = gets.to_s.to_i
x,y = Array.new(n){ gets.to_s.split.map{|t| t.to_i } }.transpose

dist = 0
cnt = 0
(0 ... n).to_a.permutation do |per|
  
  cnt += 1
  per.each_cons(2) do |i,j|
    
    dist += ( ( x[i] - x[j] ) ** 2 + ( y[i] - y[j] ) ** 2 ) ** 0.5
    
  end
end

puts dist / cnt