n = gets.to_i
vs = n.times.map {gets.split.map &:to_i}
sum = 0
(0...n).each do |i|
  (i+1...n).each do |j|
    x1, y1 = vs[i]
    x2, y2 = vs[j]
    dx, dy = x1-x2, y1-y2
    sum += Math.sqrt(dx*dx + dy*dy)
  end
end
m1 = (2..n-1).inject(1, &:*)
m2 = (2..n).inject(1, &:*)
p 2*sum*m1/m2