(defun main (&rest argv)
  (declare (ignorable argv))
  (let* ((r (read))
         (c (read))
         (rc (* r c)))
    (format t "~d~%" (if (= r c)
                         (+ (floor rc 4) (mod rc 4) -1)
                         (+ (floor rc 2) (mod rc 2) -1)))))

(main)