from collections import deque
N, M = map(int, input().split())
A = list(map(int, input().split()))
B = list(map(int, input().split()))
C = []
for a, b in zip(A, B):
    C.append(max(0, a - b))

ans = 0
que = deque()
total = 0
for i in range(N):
    c = C[i]
    que.append(c)
    total += c

    if len(que) > M:
        rm = que.popleft()
        total -= rm

    if i >= M - 1:
        if total > ans:
            ans = total
print(ans)