(defun main (&rest argv)
  (declare (ignorable argv))
  (let* ((n (read))
         (d (read))
         (x (make-array n))
         (y (make-array n))
         (p 0)
         (q -100000000000000))
    (dotimes (i n)
      (setf (aref x i) (read))
      (setf (aref y i) (read)))
    (dotimes (i n)
      (let ((new-p (+ (max p (- q d)) (aref x i)))
            (new-q (+ (max q (- p d)) (aref y i))))
        (setq p new-p)
        (setq q new-q)))
    (format t "~d~%" (max p q))))

(main)