N, M = map(int, input().split())
A = list(map(int, input().split()))
B = list(map(int, input().split()))

G = [max(0, a-b) for a, b in zip(A, B)]
S = [0]
for a in G: S.append(S[-1]+a)
range_sum = lambda l, r: S[r] - S[l]
ans = 0
for r in range(M, N+1):
    ans = max(ans, range_sum(r-M, r))
print(ans)