from itertools import accumulate
N,M=map(int,input().split())
A=list(map(int,input().split()))
B=list(map(int,input().split()))
AB=[]
for _ in range(N):
    if A[_]-B[_]>=0:
        AB.append(A[_]-B[_])
    else:
        AB.append(0)
RAB=list(accumulate(AB))
MAX=RAB[M-1]
for i in range(M,N):
    MAX=max(MAX,RAB[i]-RAB[i-M])
print(MAX)