def fact(n)
  return 1 if n <= 1
  n * fact(n - 1)
end

def dist(p, q)
  Math.sqrt(
    ((p[:x] - q[:x]) ** 2) + ((p[:y] - q[:y]) ** 2)
  )
end

def main(argv)
  n = gets.chomp.to_i
  
  ps = [nil] * n
  n.times{ |i| ps[i] = {:x => 0.0, :y => 0.0} }
  n.times do |i|
    (ps[i][:x], ps[i][:y]) = gets.chomp.split(' ').map(&:to_f)
  end
  
  ds = [nil] * n
  n.times{ |i| ds[i] = [0.0] * n }
  n.times do |i|
    n.times do |j|
      ds[i][j] = dist(ps[i], ps[j])
    end
  end
  
  sum = 0.0
  n.times do |i|
    ((i+1)...n).each do |j|
      sum += ds[i][j]
    end
  end
  
  puts (sum * fact(n - 1) * 2) / fact(n).to_f
end

if self.to_s == 'main' then
  main(ARGV)
end