(defun main (&rest argv)
  (declare (ignorable argv))
  (let* ((n (read))
         (k (read)))
    (format t "~d~%" (cond ((or (<= k 0) (> k n))
                             0)
                           ((evenp n)
                             (- n 2))
                           (t
                             (if (= k (floor (1+ n) 2))
                                 (- n 1)
                                 (- n 2)))))))

(main)