def distance(x1,y1,x2,y2)
  dx, dy=x1-x2, y1-y2
  Math.sqrt(dx**2 + dy**2)
end
N=gets.to_i
XY=N.times.map{gets.split.map(&:to_i)}

sum = 0.0
cnt = 0
(0..N-1).to_a.permutation.to_a.each do |n|
  cnt += 1
  now = n.shift
  n.each do |i|
    nextt = i
    x1, y1 = XY[now]
    x2, y2 = XY[nextt]
    sum += distance(x1,y1,x2,y2)
  end
end

puts sum/cnt