#include <iostream>
#include <cmath>
#include <vector>
#include <array>
using namespace std;

constexpr int maxniv = 18;

class rmq{
	int n;
	vector<array<int, maxniv>> d;
public:
	rmq(const vector<int>& v): n(v.size()), d(n){
		for(int i = 0; i < n; ++i){
			d[i][0] = v[i]; }
		for(int niv = 1, step = 1; niv < maxniv; ++niv, step *= 2){
			for(int i = 0; i < n; ++i){
				d[i][niv] = max(d[i][niv-1], d[min(i+step, n-1)][niv-1]); } } }
	int operator()(const int a, const int b){
		const int dist = b-a+1, niv = log2(dist), step = 1<<niv;
		return max(d[a][niv], d[b-step+1][niv]); } };

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

	vector<int> v(n, 0);
	for(auto& x : v){
		cin >> x; }
	rmq r(v);

	long long rez = v[0] + v[1];

	for(int niv = 0; niv < maxniv; ++niv){
		for(int i = 0, j = (2<<niv)-1; j < n; ++i, ++j){
			rez = max(rez, (long long)v[i] + (long long)r(i+1, j) - niv); } }

	cout << rez << endl;

	return 0; }