N, D = map(int, input().split())
dpt = [0 for i in range(N)]
dpk = [0 for i in range(N)]

dpt[0], dpk[0] = map(int, input().split())
dpk[0] = dpk[0] - D


for i in range(N-1):
    T, K = map(int, input().split())
    dpt[i + 1] = max(dpt[i] + T, dpk[i] + T - D)
    dpk[i + 1] = max(dpk[i] + K, dpt[i] + K - D)

print(max(dpk[-1],dpt[-1]))