import sys
from heapq import heappush, heappop

read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines

N, K = map(int, readline().split())
A = [0] + list(map(int, readline().split()))
B = [0] + list(map(int, readline().split()))

if N == 1:
    print(0)
    exit()

q = [(A[1] + B[2], 2)]
done = [0] * (N + 1)
done[1] = 1
ans = 0
while q:
    t, j = heappop(q)
    if done[j]:
        continue
    if j in (N - 1, N):
        ans = max(ans, t)
    done[j] = 1
    # j+1 への伝搬
    if j < N:
        t1, j1 = t + A[j] + B[j + 1], j + 1
        heappush(q, (t1, j1))
    # j+2 への伝搬
    if j < N - 1:
        t1, j1 = t + A[j] + B[j + 2] + K, j + 2
        heappush(q, (t1, j1))

print(ans)