#!/usr/bin/env python3

from sys import setrecursionlimit, stdin
from typing import Iterable

INF: int = 2**62
MOD: int = 10**9 + 7

setrecursionlimit(10**6)


def inputs(type_=int):
    ins = input().split(' ')
    ins = [x for x in ins if x != '']

    if isinstance(type_, Iterable):
        return [t(x) for t, x in zip(type_, ins)]
    else:
        return list(map(type_, ins))


def input_(type_=int):
    a, = inputs(type_)
    return a


inputi = input_


def inputstr():
    return input_(str)


# b/aの切り上げ
def ceil(b, a):
    return (a + b - 1) // a


def answer(res) -> None:
    print(res)
    exit()


def compute():
    return


def main():
    n, d = inputs()
    n += 1
    xs = [None] * n

    for i in range(n):
        xs[i] = inputs()
    xs = [[0, 0]] + xs

    dp = [[0, 0] for _ in range(n)]
    dp[0][0] = 0
    dp[0][1] = -INF
    for i in range(1, n):
        for s in range(2):
            dp[i][s] = max(dp[i-1][s] + xs[i][s], dp[i-1][1-s] + xs[i][s] - d)

    # from pprint import pprint
    # pprint(dp)
    print(max(dp[-1]))


if __name__ == '__main__':
    main()