N, M = map(int,input().split())
a = list(map(int,input().split()))
b = list(map(int,input().split()))
li = [a[i] - b[i] for i in range(N)]
souwa = [0]
for i in range(N):
    if(li[i] < 0):
        li[i] = 0
    souwa.append(souwa[-1] + li[i])
ans = 0
for i in range(N - M + 1):
    ans = max(ans, souwa[i + M] - souwa[i])
print(ans)