(defun solve (stream)
  (let ((wanted (make-array 2021 :element-type 'bit :initial-element 0)))
	(loop
	  for x = (parse-integer (read-line stream))
	  for y = (- 2020 x)
	  if (zerop (elt wanted x))
		do (setf (elt wanted y) 1)
	  else
		return (* x y))))

(defun solve2 (stream)
  (let ((wanted (make-hash-table :test 'eql :size 1010))
		(prior nil))
	(loop
	  for x = (parse-integer (read-line stream))
	  for product = (gethash x wanted)
	  for max-y = (- 2020 x)
	  when product
		return (* product x)
	  do (loop
		   for y in prior
		   while (<= y max-y)
		   do (setf (gethash (- max-y y) wanted) (* x y)))
	  (setf prior (merge 'list prior (list x) #'<)))))