#include<bits/stdc++.h>
using namespace std;
#define FOR(i,a,b) for(int i = (a); i <= (b); ++i)
#define FORD(i,a,b) for(int i = (a); i >= (b); --i)
#define RI(i,n) FOR(i,1,(n))
#define REP(i,n) FOR(i,0,(n)-1)
#define mini(a,b) a=min(a,b)
#define maxi(a,b) a=max(a,b)
#define mp make_pair
#define pb push_back
#define st first
#define nd second
#define sz(w) (int) w.size()
typedef vector<int> vi;
typedef long long ll;
typedef long double ld;
typedef pair<int,int> pii;
const int inf = 2e9 + 5;
const int nax = 1e5 + 5;
int t[nax];
int dp[nax][20];

int main() {
	int n;
	scanf("%d", &n);
	RI(i, n) scanf("%d", &t[i]);
	int ans = -inf;
	REP(i, n + 2) REP(k, 20) dp[i][k] = -inf;
	FOR(i, 2, n) {
		int j = 2;
		dp[i][0] = t[i-1];
		maxi(ans, t[i-1] + t[i]);
		RI(k, 20) {
			dp[i][k] = dp[i][k-1];
			if(i - j >= 1) {
				maxi(dp[i][k], dp[i-j][k-1]);
				maxi(dp[i][k], t[i-j]);
			}
			maxi(ans, t[i] + dp[i][k] - k);
			j *= 2;
		}
	}
	printf("%d\n", ans);
	return 0;
}