n, fare = map(int, input().split())
revenues = [list(map(int, input().split())) for _ in range(n)]

# print(n)
# print(fare)
# print(revenues)

t_answers = [0] * n
k_answers = [0] * n

t_answers[0] = revenues[0][0]
k_answers[0] = revenues[0][1] - fare

for i in range(1, n):
    t_answers[i] = revenues[i][0] + max(t_answers[i - 1], k_answers[i - 1] - fare)
    k_answers[i] = revenues[i][1] + max(t_answers[i - 1] - fare, k_answers[i - 1])

# print(t_answers)
# print(k_answers)

print(max(t_answers[n - 1], k_answers[n - 1]))