N, D = map(int, input().split())
dp_t, dp_k = 0, -10 ** 8
for _ in range(N):
    t, k = map(int, input().split())
    dp_t, dp_k = max(dp_t + t, dp_k + t - D), max(dp_k + k, dp_t + k - D)
print(max(dp_t, dp_k))