#include <cstdio>
#include <cmath>
#include <algorithm>
using namespace std;
int v[100100];
int d[100100][17];
int n;

void pre()
{
    int i,j;
    int lim = log2(100100);
    for (j = 1; j <= lim;j++)
    {
        int m = n-(1<<j)+1;
        int put = (1<<j);
        for (int i = 0; i <= m; i++)
            d[i][j] = max(d[i][j-1],d[i+put/2][j-1]);
    }
}

int main()
{
    //freopen("input.txt","r",stdin);

    scanf("%d",&n);
    for (int i = 1;i <= n; i++)
    {
        scanf("%d",&v[i]);
        d[i][0] = v[i];
    }
    pre();

    int ans = -(1<<31);
    for (int i = 1;i < n; i++)
    {
        int p = 2, p2 = min(p*2-1, n),add=1;
        ans = max(ans, v[i]+v[i+1]);
        while (p <=n)
        {
            ans = max( ans, v[i]+ d[i+p][add] - add);
            p = p*2;
            add++;
            p2 = min(p*2-1, n);
        }
    }

    printf("%d",ans);

    return 0;
}