require 'pp'

def calc(p1, p2)
  x = p1[0]-p2[0]
  y = p1[1]-p2[1]
  Math.sqrt(x*x+y*y)
end

def permutation(a, x, xx)
  if a.size == 0
    return xx << x
  end

  a.each.with_index do |c, i|
    b = a.dup
    b.delete_at(i)
    permutation(b, x + [c], xx)
  end
end

def factr(n)
  (1..n).reduce(&:*)
end

def main(a, n)
  f = factr(n)

  xx = []
  permutation(a, [], xx)

  total = 0
  xx.each do |x|
    sum = 0
    x.each_cons(2) do |a, b|
      sum += calc(a, b)
    end
    total += sum / f
  end

  puts total
end

n = gets.chop.to_i
a = []
n.times { a << gets.chomp.split.map(&:to_i) }
main(a, n)