(defun d2p1 (file)
  (loop with x = 0 and y = 0
        for (direction amount) on (read-values file) by #'cddr
        do (ecase direction
             (forward (incf x amount))
             (up (setf y (max 0 (- y amount))))
             (down (incf y amount)))
        finally (return (* x y))))