#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)

int main() {
    cin.tie(nullptr);
    ios_base::sync_with_stdio(false);
    int n, m;
    cin >> n >> m;
    vector<ll> a(n), b(n);
    vector<ll> c(n + 1, 0);
    rep(i, n) cin >> a[i];
    rep(i, n) cin >> b[i];
    rep(i, n) c[i + 1] += a[i] - b[i], c[i + 1] = max(c[i + 1], 0LL);
    rep(i, n) c[i + 1] += c[i];

    ll ans = 0;
    rep(i, n - m + 1) {
        ans = max(ans, c[i + m] - c[i]);
    }
    cout << ans << endl;
    return 0;
}