import logging
import sys
from inspect import currentframe

sys.setrecursionlimit(10 ** 6)
input = sys.stdin.readline

logging.basicConfig(level=logging.DEBUG)


def dbg(*args):
    id2names = {id(v): k for k, v in currentframe().f_back.f_locals.items()}
    logging.debug(
        ", ".join(id2names.get(id(arg), "???") + " = " + repr(arg) for arg in args)
    )


def main():

    n, d = map(int, input().split())
    t = [0] * n
    k = [0] * n
    for i in range(n):
        t[i], k[i] = map(int, input().split())
    # dp = [[0] * (n + 1) for _ in range(2)]
    dp = [[0] * 2 for _ in range(n + 1)]
    dp[0][1] = -float("inf")
    for i in range(n):
        dp[i + 1][0] = max(dp[i][0] + t[i], dp[i][1] + t[i] - d)
        dp[i + 1][1] = max(dp[i][1] + k[i], dp[i][0] + k[i] - d)
    ans = max(dp[n][0], dp[n][1])
    dbg(dp)
    print(ans)


if __name__ == "__main__":
    main()