from itertools import accumulate
N,M = list(map(int,input().split()))
A = list(map(int,input().split()))
B = list(map(int,input().split()))
C = [max(A[i] - B[i],0) for i in range(N)]
S = [0] + list(accumulate(C))

ans = 0
for i in range(N-M+1):
    ans = max(ans, S[i+M] - S[i])
print(ans)