#!/usr/bin/env python3

import sys
# import time
# import math
# import numpy as np
# import scipy.sparse.csgraph as cs            # csgraph_from_dense(ndarray, null_value=inf), bellman_ford(G, return_predecessors=True), dijkstra, floyd_warshall
# import random                                # random, uniform, randint, randrange, shuffle, sample
# import string                                # ascii_lowercase, ascii_uppercase, ascii_letters, digits, hexdigits
# import re                                    # re.compile(pattern) => ptn obj; p.search(s), p.match(s), p.finditer(s) => match obj; p.sub(after, s)
# from bisect import bisect_left, bisect_right # bisect_left(a, x, lo=0, hi=len(a)) returns i such that all(val<x for val in a[lo:i]) and all(val>-=x for val in a[i:hi]).
# from collections import deque                # deque class. deque(L): dq.append(x), dq.appendleft(x), dq.pop(), dq.popleft(), dq.rotate()
# from collections import defaultdict          # subclass of dict. defaultdict(facroty)
# from collections import Counter              # subclass of dict. Counter(iter): c.elements(), c.most_common(n), c.subtract(iter)
# from datetime import date, datetime          # date.today(), date(year,month,day) => date obj; datetime.now(), datetime(year,month,day,hour,second,microsecond) => datetime obj; subtraction => timedelta obj
# from datetime.datetime import strptime       # strptime('2019/01/01 10:05:20', '%Y/%m/%d/ %H:%M:%S') returns datetime obj
# from datetime import timedelta               # td.days, td.seconds, td.microseconds, td.total_seconds(). abs function is also available.
# from copy import copy, deepcopy              # use deepcopy to copy multi-dimentional matrix without reference
# from functools import reduce                 # reduce(f, iter[, init])
# from functools import lru_cache              # @lrucache ...arguments of functions should be able to be keys of dict (e.g. list is not allowed)
# from heapq import heapify, heappush, heappop # built-in list. heapify(L) changes list in-place to min-heap in O(n), heappush(heapL, x) and heappop(heapL) in O(lgn).
# from heapq import nlargest, nsmallest        # nlargest(n, iter[, key]) returns k-largest-list in O(n+klgn).
# from itertools import count, cycle, repeat   # count(start[,step]), cycle(iter), repeat(elm[,n])
# from itertools import groupby                # [(k, list(g)) for k, g in groupby('000112')] returns [('0',['0','0','0']), ('1',['1','1']), ('2',['2'])]
# from itertools import starmap                # starmap(pow, [[2,5], [3,2]]) returns [32, 9]
# from itertools import product, permutations  # product(iter, repeat=n), permutations(iter[,r])
# from itertools import combinations, combinations_with_replacement
from itertools import accumulate             # accumulate(iter[, f])
# from operator import itemgetter              # itemgetter(1), itemgetter('key')
# from fractions import gcd                    # for Python 3.4 (previous contest @AtCoder)



def main():
    mod = 1000000007                # 10^9+7
    inf = float('inf')              # sys.float_info.max = 1.79...e+308
    # inf = 2 ** 64 - 1             # (for fast JIT compile in PyPy) 1.84...e+19
    sys.setrecursionlimit(10**6)    # 1000 -> 1000000
    def input(): return sys.stdin.readline().rstrip()
    def ii():    return int(input())
    def mi():    return map(int, input().split())
    def mi_0():  return map(lambda x: int(x)-1, input().split())
    def lmi():   return list(map(int, input().split()))
    def lmi_0(): return list(map(lambda x: int(x)-1, input().split()))
    def li():    return list(input())
    
    
    n, c = mi()
    place = []
    inv_place = []
    value = []
    for _ in range(n):
        x, v = mi()
        place.append(x)
        inv_place.append(c - x)
        value.append(v)
    accum_val = list(accumulate(value))
    counter_accum_val = list(reversed(list(accumulate(reversed(value)))))

    clockwise = [[] for _ in range(n)]
    counter_clockwise = [[] for _ in range(n)]

    for i in range(n):
        clockwise[i] = [accum_val[i] - place[i], accum_val[i] - 2 * place[i]]
        counter_clockwise[i]= [counter_accum_val[i] - inv_place[i], counter_accum_val[i] - 2 * inv_place[i]]
    
    # import pprint
    # print(clockwise)
    # print(counter_clockwise)
    # print('')

    # 時計回りのみ -> clockwise の [0] の max
    # 反時計回りのみ -> counter_clockwise の [0] の max
    max_memo = - inf
    max_memo_clock_0 = [0] * n
    for i in range(n):
        max_memo = max(max_memo, clockwise[i][0])
        max_memo_clock_0[i] = max_memo
    

    max_memo = - inf
    max_memo_counter_clock_0 = [0] * n
    for i in range(n-1, -1, -1):
        max_memo = max(max_memo, counter_clockwise[i][0])
        max_memo_counter_clock_0[i] = max_memo

    
    # print(max_memo_clock_0)
    # print(max_memo_counter_clock_0)

    ans = max(max(max_memo_clock_0), max(max_memo_counter_clock_0), 0)
    # print(ans)

    for i in range(n-1, 0, -1):
        ans = max(ans, counter_clockwise[i][1] + max_memo_clock_0[i-1])
    for i in range(n-1):
        ans = max(ans, clockwise[i][1] + max_memo_counter_clock_0[i+1])

    
    print(ans)



if __name__ == "__main__":
    main()


Measured memory_bytes: 86876