n,m=map(int,input().split())
a=list(map(int,input().split()))
b=list(map(int,input().split()))
for i in range(n):
	a[i]-=b[i]
	a[i]=max(a[i],0)
x=0
for i in range(m):
	x+=a[i]
ans=x 
for i in range(m,n):
	x+=a[i]-a[i-m]
	ans=max(ans,x)
print(ans)