#include <iostream>
#include <cstdio>
#include <cmath>
#include <algorithm>
#include <cstring>
#include <string>
#include <vector>
#include <set>
#include <map>
#include <ctime>
using namespace std;

const int N = 1e5 + 5;
int a[N], a0[N], lg[N];
map<long long, int> mp;

int main() {
    int n;
    cin >> n;
    lg[0] = lg[1] = 0;
    for(int i = 2; i < N; ++i)
        lg[i] = lg[i/2] + 1;
    
    for(int i = 0; i < n; ++i) {
        scanf("%d", &a[i]);
        a0[i] = a[i];
    }
    sort(a0, a0+n);
    long long rd = a0[n-1] + a0[n-2], ld = rd - 32;
    long long ans = -1e12;
    
    for(int i = 0; i < n; ++i) {
        for(long long j = ld - a[i]; j <= rd - a[i]; ++j) {
            if(mp.count(j)) {
                int ii = mp[j];
                long long s = a[ii] + a[i] - lg[i-ii];
                ans = max(ans, s);
            }
        }
        mp[a[i]] = i;
    }
    
    cout << ans << endl;
    
    
    
    
    
    return 0;
}