n = gets.to_i
x = []
n.times do
  x << gets.split.map(&:to_i)
end
y = Array.new(n,0)
0.upto(n-1) do |i|
  y[i] = i+1
end
com = y.permutation(n).to_a
count = 0
com.each_with_index do |i|
  0.upto(i.length-2) do |num|
    c = (x[(i[num+1]-1)][0]-x[(i[num]-1)][0])**2
    d = (x[(i[num+1]-1)][1]-x[(i[num]-1)][1])**2
    count+=Math.sqrt(c+d)
  end
end
puts count/com.length