(define (poky L K)
  (let ((n (quotient L 2)))
    (cond [(<= (truncate (/ n K)) 1) 0]
          [(= (modulo n K) 0) (* (- (truncate (/ n K)) 1) K)]
          [else (* (truncate (/ n K)) K)])))

(define (main args)
  (let* ((L (read))
         (K (read)))
    (print (poky L K))) 0)