N, M = map(int,input().split())
A = list(map(int,input().split()))
B = list(map(int,input().split()))
S = [0]
for (a,b) in zip(A,B):
	nxt = S[-1] + max(a-b, 0)
	S.append(nxt)
	
ans = 0
for i in range(N-M+1):
	tmp = S[i+M] - S[i]
	ans = max(ans, tmp)
	
print(ans)