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

template<class T> istream& operator >> (istream& is, vector<T>& vec) {
    for(T& x : vec) is >> x;
    return is;
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    int n, m;
    cin >> n >> m;
    vector<int> a(n);
    for(int i = 0; i < n; i++) cin >> a[i];
    vector<ll> sv(n + 1);
    ll ans = 0;
    for(int i = 0, v; i < n; i++){
        cin >> v;
        sv[i + 1] = sv[i] + max(0, a[i] - v);
        if(i + 1 >= m) ans = max(ans, sv[i + 1] - sv[i + 1 - m]);
    }
    cout << ans << "\n";
}