;; -*- coding: utf-8 -*-
(eval-when (:compile-toplevel :load-toplevel :execute)
  (defparameter OPT
    #+swank '(optimize (speed 3) (safety 2))
    #-swank '(optimize (speed 3) (safety 0) (debug 0)))
  #+swank (progn (ql:quickload '(:cl-debug-print :fiveam))
                 (shadow :run)
                 (use-package :fiveam)))
#+swank (cl-syntax:use-syntax cl-debug-print:debug-print-syntax)

;; BEGIN_INSERTED_CONTENTS
(declaim (ftype (function * (values fixnum &optional)) read-fixnum))
(defun read-fixnum (&optional (in *standard-input*))
  (declare #.OPT)
  (macrolet ((%read-byte ()
               `(the (unsigned-byte 8)
                     #+swank (char-code (read-char in nil #\Nul))
                     #-swank (sb-impl::ansi-stream-read-byte in nil #.(char-code #\Nul) nil))))
    (let* ((minus nil)
           (result (loop (let ((byte (%read-byte)))
                           (cond ((<= 48 byte 57)
                                  (return (- byte 48)))
                                 ((zerop byte) ; #\Nul
                                  ;; (return-from read-fixnum 0)
                                  (error "Read EOF or #\Nul."))
                                 ((= byte #.(char-code #\-))
                                  (setf minus t)))))))
      (declare ((integer 0 #.most-positive-fixnum) result))
      (loop
        (let* ((byte (%read-byte)))
          (if (<= 48 byte 57)
              (setq result (+ (- byte 48) (* 10 (the (integer 0 #.(floor most-positive-fixnum 10)) result))))
              (return (if minus (- result) result))))))))

(defmacro dbg (&rest forms)
  #+swank
  (if (= (length forms) 1)
      `(format *error-output* "~A => ~A~%" ',(car forms) ,(car forms))
      `(format *error-output* "~A => ~A~%" ',forms `(,,@forms)))
  #-swank (declare (ignore forms)))

(defmacro define-int-types (&rest bits)
  `(progn
     ,@(mapcar (lambda (b) `(deftype ,(intern (format nil "UINT~A" b)) () '(unsigned-byte ,b))) bits)
     ,@(mapcar (lambda (b) `(deftype ,(intern (format nil "INT~A" b)) () '(signed-byte ,b))) bits)))
(define-int-types 2 4 7 8 15 16 31 32 62 63 64)

(declaim (inline println))
(defun println (obj &optional (stream *standard-output*))
  (let ((*read-default-float-format* 'double-float))
    (prog1 (princ obj stream) (terpri stream))))

(defconstant +mod+ 1000000007)

;; Body

(defun solve (xs vs c)
  (declare #.OPT
           ((simple-array uint62 (*)) xs)
           ((simple-array uint32 (*)) vs))
  (let* ((n (length xs))
         (cumuls-clock (make-array n :element-type 'fixnum))
         (cumuls-anticlock (make-array n :element-type 'fixnum))
         (dp-clock (make-array n :element-type 'fixnum))
         (dp-anticlock (make-array n :element-type 'fixnum)))
    (setf (aref cumuls-clock 0) (- (aref vs 0) (aref xs 0)))
    (loop for i from 1 below n
          do (setf (aref cumuls-clock i)
                   (+ (aref cumuls-clock (- i 1))
                      (aref vs i)
                      (- (aref xs (- i 1)) (aref xs i)))))
    (setf (aref cumuls-anticlock (- n 1)) (- (aref vs (- n 1))
                                             (* 2 (- c (aref xs (- n 1))))))
    (loop for i from (- n 2) downto 0
          do (setf (aref cumuls-anticlock i)
                   (+ (aref cumuls-anticlock (+ i 1))
                      (aref vs i)
                      (- (* 2 (- (aref xs (+ i 1)) (aref xs i)))))))
    (setf (aref dp-clock 0) (aref cumuls-clock 0))
    (loop for i from 1 below n
          do (setf (aref dp-clock i)
                   (max (aref dp-clock (- i 1)) (aref cumuls-clock i))))
    (setf (aref dp-anticlock (- n 1)) (aref cumuls-anticlock (- n 1)))
    (loop for i from (- n 2) downto 0
          do (setf (aref dp-anticlock i)
                   (max (aref dp-anticlock (+ i 1)) (aref cumuls-anticlock i))))
    (max (reduce #'max cumuls-clock)
         (loop for i from 0 below (- n 1)
               maximize (+ (aref dp-clock i) (aref dp-anticlock (+ i 1)))))))

(defun main ()
  (declare #.OPT)
  (let* ((n (read))
         (c (read))
         (xs (make-array n :element-type 'uint62))
         (rev-xs (make-array n :element-type 'uint62))
         (vs (make-array n :element-type 'uint32))
         (rev-vs (make-array n :element-type 'uint32)))
    (declare (uint62 c) (uint32 n))
    (dotimes (i n)
      (let ((x (read-fixnum))
            (v (read-fixnum)))
        (setf (aref xs i) x)
        (setf (aref rev-xs (- n i 1)) (- c x))
        (setf (aref vs i) v)
        (setf (aref rev-vs (- n i 1)) v)))
    (println (max 0 (solve xs vs c) (solve rev-xs rev-vs c)))))

#-swank(main)


Measured memory_bytes: 26852