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

C = []

for i in range(N):
	C.append(max(A[i]-B[i], 0))

val = sum(C[:M])
ans = sum(C[:M])

for i in range(N-M):
	val += C[M+i] - C[i]
	ans = max(ans, val)

print(ans)