/*
*/

//#pragma comment(linker, "/STACK:16777216")
#define _CRT_SECURE_NO_WARNINGS

#include <fstream>
#include <iostream>
#include <string>
#include <complex>
#include <math.h>
#include <set>
#include <vector>
#include <map>
#include <queue>
#include <stdio.h>
#include <stack>
#include <algorithm>
#include <list>
#include <ctime>
#include <memory.h>
#include <assert.h>

#define y0 sdkfaslhagaklsldk
#define y1 aasdfasdfasdf
#define yn askfhwqriuperikldjk
#define j1 assdgsdgasghsf
#define tm sdfjahlfasfh
#define lr asgasgash

#define eps 1e-9
#define M_PI 3.141592653589793
#define bs 1000000007
#define bsize 512

const int N = (1 << 18);

using namespace std;

int n;
long long ar[N];
long long t[N * 4];
long long ans;

void build(int v, int tl, int tr)
{
	if (tl == tr)
	{
		t[v] = ar[tl];
		return;
	}
	int tm = tl + tr;
	tm /= 2;
	build(v * 2, tl, tm);
	build(v * 2 + 1, tm + 1, tr);
	t[v] = max(t[v * 2], t[v * 2 + 1]);
}

long long get(int v, int tl, int tr, int l, int r)
{
	if (l > r)
		return -1e18;
	if (l == tl&&r == tr)
		return t[v];
	int tm = tl + tr;
	tm /= 2;
	return max(get(v * 2, tl, tm, l, min(r, tm)), get(v * 2 + 1, tm + 1, tr, max(tm + 1, l), r));
}

int main(){
	//freopen("route.in","r",stdin);
	//freopen("route.out","w",stdout);
	//freopen("F:/in.txt", "r", stdin);
	//freopen("F:/output.txt", "w", stdout);
	ios_base::sync_with_stdio(0);
	//cin.tie(0);
	
	cin >> n;
	for (int i = 0; i < n; i++)
	{
		cin >> ar[i];
	}

	build(1, 0, n - 1);

	ans = -1e18;

	for (int cl = 0; cl <= 17; cl++)
	{
		int mdist = (1 << cl) * 2 - 1;

//		cout << cl << " " << mdist << endl;

		for (int i = 0; i < n; i++)
		{
			int l, r;
			l = i - mdist;
			r = i - 1;
			if (l < 0)
				l = 0;
			long long here = get(1, 0, n - 1, l, r);
			ans = max(ans, here + ar[i] - cl);
		}
	}

	cout << ans << endl;

	cin.get(); cin.get();
	return 0;
}