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

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

s = [0] * (n + 1)
for i in range(n):
    s[i + 1] = s[i] + c[i]

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

print(ans)