#include <bits/stdc++.h>
#define rep(i,n) for (int i=0; i < (int)(n); i++)
#define all(c) c.begin(), c.end()
using namespace std;
typedef long long ll; typedef long double ld;
using vi = vector<int>; using vvi = vector<vi>;
using vl = vector<ll>;  using vvl = vector<vl>;

int main() {
	ios::sync_with_stdio(0); cin.tie(0); //入出力高速化

	//事前計算と累積和
	int N,M; cin>>N>>M;
	vl A(N); rep(i,N) cin>>A[i];
	vl B(N); rep(i,N) cin>>B[i];
	vl C(N+1); C[0] = 0;
	rep(i,N) C[i+1] = max(A[i] - B[i], 0LL);
	rep(i,N) C[i+1] += C[i];

	ll ans = 0;
	rep(i,N) if(i+1-M >= 0) {
		ans = max(ans, C[i+1] - C[i+1-M]);
	}
	cout<<ans<<endl;
}