n, d = map(int, input().split())

T = 0
K = -d
for _ in range(n):
    t, k = map(int, input().split())
    T1 = max(T + t, K + t - d)
    K1 = max(K + k, T + k - d)
    T = T1
    K = K1   

print(max(T, K))