#!/usr/bin/env python3

INF: int = 2**62

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

    return list(map(type_, ins))

def main():
    n, d = inputs()

    xs = [None] * n

    for i in range(n):
        xs[i] = inputs()
    xs = [[0, 0]] + xs
    n += 1

    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()