n, m = map(int, input().split())
a = list(map(int, input().split()))
b = list(map(int, input().split()))
s = [0]
for i in range(n):
    s.append(s[-1] + max(0, a[i] - b[i]))
ans = 0
for i in range(m, n + 1):
    ans = max(ans, s[i] - s[i - m])
print(ans)