#include <bits/stdc++.h>

using namespace std;

#define ll long long
#define pb push_back
#define mp make_pair
#define pii pair<int, int>
#define pll pair<ll, ll>
#define all(x) (x).begin(), (x).end()
#define fi first
#define se second

const int nmax = 100005;

int n, a[nmax], logg[nmax];
ll sol = -(1LL << 40);
deque<int> q;

int main() {
	cin.sync_with_stdio(false);

	// freopen("a.in", "r", stdin);
	// freopen("a.out", "w", stdout);

	cin >> n;
	for (int i = 1; i <= n; i++)
		cin >> a[i];

	for (int i = 2; i <= n; i++)
		logg[i] = logg[i / 2] + 1;

	for (int lg = 0; (1 << lg) <= n; lg++) {
		int i = 1;

		q.clear();
		q.pb(1);

		for (int j = 2; j <= n; j++) {
			while (logg[j - i] > lg) {
				if (!q.empty() && q.front() == i)
					q.pop_front();
				i++;
			}

			ll mlc = a[j];
			mlc += a[q.front()];
			mlc -= lg;
			sol = max(sol, mlc);

			while (!q.empty() && a[q.back()] <= a[j])
				q.pop_back();
			q.pb(j);
		}
	}

	cout << sol;

	return 0;
}