n, d = map(int, input().split())
days = [tuple(map(int, input().split())) for _ in range(n)]

prev_t, prev_k = 0, float('-inf')

for t, k in days:
    current_t = max(prev_t + t, prev_k + (t - d))
    current_k = max(prev_t + (k - d), prev_k + k)
    prev_t, prev_k = current_t, current_k

print(max(prev_t, prev_k))