#include <bits/stdc++.h>
using namespace std;

int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);

    int N,M; cin >> N >> M;
    vector<long long> A(N),B(N);
    for(auto &a : A) cin >> a;
    for(auto &b : B) cin >> b;
    for(int i=0; i<N; i++) A.at(i) -= B.at(i),A.at(i) = max(0LL,A.at(i));
    long long answer = 0,now = 0;
    for(int i=0; i<M-1; i++) now += A.at(i);
    for(int i=M-1; i<N; i++){
        now += A.at(i);
        answer = max(answer,now);
        now -= A.at(i+1-M);
    }
    cout << answer << endl;
}