N, M = map(int, input().split())
A = list(map(int, input().split()))
B = list(map(int, input().split()))

R = [0 for _ in range(N+1)]
for i in range(1, N+1):
    if A[i-1] - B[i-1] >0:
        R[i] = R[i-1] + A[i-1] - B[i-1]
    else:
        R[i] = R[i-1]
ANS = 0

for j in range( N-M+1 ):
    ANS = max(ANS, R[j+M] - R[j])

print(ANS)