N,M=map(int,input().split())
A=list(map(int,input().split()))
B=list(map(int,input().split()))
C=[max(0,A[i]-B[i]) for i in range(N)]
cur=sum(C[:M])
ans=cur
for i in range(M,N):
    cur+=C[i]
    cur-=C[i-M]
    ans=max(ans,cur)
print(ans)