# -*- coding: utf-8 -*-
"""
No.561 東京と京都
https://yukicoder.me/problems/no/561

"""
import sys
from sys import stdin
input = stdin.readline


def solve(earnings, D):
    TOKYO, KYOTO = 0, 1
    res = [[0, -D]]

    for t, k in earnings:
        tokyo_e = max(res[-1][TOKYO] + t, res[-1][KYOTO] + t - D)
        kyoto_e = max(res[-1][TOKYO] + k - D, res[-1][KYOTO] + k)
        res.append([tokyo_e, kyoto_e])
    return max(res[-1][TOKYO], res[-1][KYOTO])


def main(args):
    N, D = map(int, input().split())
    earnings = []
    for _ in range(N):
        tokyo, kyoto = map(int, input().split())
        earnings.append([tokyo, kyoto])
    ans = solve(earnings, D)
    print(ans)


if __name__ == '__main__':
    main(sys.argv[1:])