N,M = map(int,input().split())
A = list(map(int,input().split()))
B = list(map(int,input().split()))
C = [max(0,a-b) for a,b in zip(A,B)]

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