N, M = map(int, input().split())
A = list(map(int, input().split()))
B = list(map(int, input().split()))
C = []
for i in range(N):
    v = max(0, A[i] - B[i])
    C.append(v)
ans = 0
now = sum(C[:M])
ans = now
for i in range(M, N):
    now -= C[i-M]
    now += C[i]
    ans = max(ans, now)
print(ans)