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

price = [max(0, a[i] - b[i]) for i in range(n)]

prefix = [0]
for i in range(n):
    prefix.append(prefix[-1] + price[i])

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