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

C = [0] * (N + 1)
for i in range(N):
	C[i + 1] = C[i] + max(0, A[i] - B[i])

print(max(C[i + M] - C[i] for i in range(N - M + 1)))