N, K = map(int, input().split())
A = list(map(int, input().split()))
B = list(map(int, input().split()))
get = []
for i in range(N):
    get.append(A[i] - B[i])
tmp = 0
for i in range(K):
    if 0 <= get[i]:
        tmp += get[i]
ans = tmp
for i in range(N - K):
    if 0 <= get[i + K]:
        tmp += get[i + K]
    if 0 <= get[i]:
        tmp -= get[i]
    ans = max(ans, tmp)
print(ans)