task D

#include <cstdio>
#include <cstdlib>
#include <iostream>
#include <set>
#include <vector>
#include <map>
#include <algorithm>
#include <string>
#include <cstring>
#include <cmath>
#include <fstream>
#include <ctime>
#include <queue>

#define X first
#define Y second
#define pii pair<int,int>
#define ll long long

using namespace std;

int n;
vector<int> a;
int ans;

void rec(vector<int> x)
{
    int sum=0;
    //cout<<x.size();
    for (int i=0;i<(int)x.size(); i++)
    {
        sum += x[i];
        //cout<<" "<<x[i];
    }
    //cout<<" "<<sum<<endl;
    ans = max(sum, ans);
    if ( (x.size()%2!=0) || (x.size()<6))
        return;
    vector<int> t1, t2;
    t1.resize(x.size()/2); t2.resize(x.size()/2);
    for (int i=0; i<(int)x.size(); i++)
    {
        if ( i%2==0)
        {
            t1[i/2]=x[i];
        }
        else
        {
            t2[i/2]=x[i];
        }
    }
    rec(t1);
    rec(t2);
}

int main()
{
    scanf("%d", &n);
    a.resize(n);
    for (int i=0; i<n; i++)
    {
        scanf("%d", &a[i]);
    }
    ans=-20000*1000;
    rec(a);
    cout<<ans;
    return 0;
}