N,M=map(int,input().split())
A=list(map(int,input().split()))
B=list(map(int,input().split()))
v=[0]*(N+1)
for i in range(N):
  v[i]=max(0,A[i]-B[i])
  v[i]+=v[i-1]
result=0
for i in range(M-1,N):
  result=max(result,v[i]-v[i-M])
print(result)