n, k = map(int, input().split())
A = list(map(int, input().split()))
B = list(map(int, input().split()))

if n == 0:
    print(0)
    exit()

t = [0] * n

if n >= 2:
    t[1] = t[0] + A[0] + B[1]

for j in range(3, n + 1):
    idx = j - 1
    option1 = t[j - 2] + A[j - 2] + B[idx]
    option2 = t[j - 3] + A[j - 3] + B[idx] + k
    t[idx] = min(option1, option2)

max_time = max(t)
print(max_time)