(defun main (&rest argv)
  (declare (ignorable argv))
  (let* ((n (read))
         (m (read)))
    (format t "~d~%" (cond
                      ((and (> n m) (evenp n) (<= (floor n 2) m))
                        2)
                      ((<= n m)
                        1)
                      (t
                        -1)))))

(main)