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

int main() {
    int n,m;
    cin >> n >> m;

    vector<ll>num (n);
    ll now = 0;

    for(int i=0;i<n;i++){
        cin >> num[i];
        if(num[i] < 0) num[i] = 0;

        if(i < m) now += num[i];
    }

    ll ans = now;
    for(int i=0;i+m<n;i++){
        now -= num[i];
        now += num[i+m];

        ans = max(ans,now);
    }

    cout << ans << endl;
}