import math
import sys


def input(): return sys.stdin.readline().rstrip('\n')


def main():
    n, d = map(int, input().split())
    tx, kx = 0, -d
    for i in range(n):
        t, k = map(int, input().split())
        tx, kx = max(tx+t, kx+t-d), max(kx+k, tx+k-d)

    print(max(tx, kx))


if __name__ == '__main__':
    main()