def main():
    n, m = map(int, input().split())
    a = list(map(int, input().split()))
    b = list(map(int, input().split()))

    d = [max(a[i] - b[i], 0) for i in range(n)]
    cumsum = [0]
    for ai in d:
        cumsum.append(cumsum[-1] + ai)

    ans = 0
    for i in range(n - m + 1):
        ans = max(ans, cumsum[i + m] - cumsum[i])

    print(ans)


if __name__ == "__main__":
    main()