def main():
    N, D = map(int, input().split())
    benefits = [tuple(map(int, input().split())) for _ in range(N)]

    benefits_table = [[0, 0] for _ in range(N+1)]
    benefits_table[0][1] = -D

    for day in range(1, N+1):
        benefits_table[day][0] = max(
            benefits_table[day-1][0]+benefits[day-1][0],
            benefits_table[day-1][1]+benefits[day-1][0]-D
        )
        benefits_table[day][1] = max(
            benefits_table[day-1][1]+benefits[day-1][1],
            benefits_table[day-1][0]+benefits[day-1][1]-D
        )

    print(max(benefits_table[N]))


if __name__ == "__main__":
    main()