(defun main (&rest argv)
  (declare (ignorable argv))
  (let* ((n (read))
         (res 0))
    (when (zerop (mod n 3))
          (format t "3~%")
          (return-from main))
    (when (zerop (mod n 4))
          (format t "4~%")
          (return-from main))
    (loop for i from 5 to (isqrt n) by 2
          when (zerop (mod n i))
            do (setq res i)
               (return))
    (cond ((and (evenp n) (zerop res))
            (setq res (floor n 2)))
          ((zerop res)
            (setq res n))
          (t))  
    (format t "~d~%" res)))

(main)