#include <bits/stdc++.h>

using namespace std;

typedef long long ll;

int main() {


    int n, k;
    cin >> n >> k;

    vector < ll > a(n), pmax(n);

    for (auto & x: a)
        cin >> x;

    pmax[0] = a[0];

    for (int i = 1; i < n; i++)
        pmax[i] = max(a[i], a[i] + pmax[i - 1]);

    ll sm = 0;

    for (int i = 0; i < k; i++)
        sm += a[i];

    ll ans = sm;

    for (int i = k; i < n; i++) {
        sm += a[i] - a[i - k];

        ll cur = sm;

        if (pmax[i - k] > 0)
            cur += pmax[i - k];

        ans = max(ans, cur);
    }

    cout << max(0LL, ans) << endl;

    return 0;
}