N, D = map(int, input().split())
TK = [list(map(int, input().split())) for _ in range(N)]
TK.reverse()

t, k = TK.pop()
tokyo = t
kyoto = max(0, k - D)
while TK:
    t, k = TK.pop()
    tokyo, kyoto = max(tokyo, kyoto - D) + t, max(kyoto, tokyo - D) + k

print(max(tokyo, kyoto))