#include <iostream>
using namespace std;

struct iostream_init_struct
{ 
	iostream_init_struct()
	{
		std::cin.tie(0);
		std::cin.sync_with_stdio(false);
	}
} iostream_init;

#include <queue>
#include <set>
#include <utility>
#include <algorithm>

int N;
int A[2000];

enum Position
{
	LEFT,
	RIGHT
};

class Agent
{
public:
	int time;
	int left;
	int right;
	Position place;
	Agent(int time, int left, int right, Position place) :
		time(time), left(left), right(right), place(place) {}

	Agent nextRight() const
	{
		int next_time;
		int next_left = left;
		int next_right = right - 1;
		Position next_place = RIGHT;

		if (place == RIGHT)
		{
			next_time = max(time, A[right]) + 1;
		}
		else
		{
			next_time = max(time + right - left, A[right]) + 1;
		}

		return Agent(next_time, next_left, next_right, next_place);
	}

	Agent nextLeft() const
	{
		int next_time;
		int next_left = left + 1;
		int next_right = right;
		Position next_place = LEFT;

		if (place == RIGHT)
		{
			next_time = max(time + right - left, A[left]) + 1;
		}
		else
		{
			next_time = max(time, A[left]) + 1;
		}

		return Agent(next_time, next_left, next_right, next_place);
	}

	bool finished() const
	{
		return left >= right;
	}

	int finalTime() const
	{
		return max(time, A[left]);
	}

	int toInt() const
	{
		return left + 10000 * (right + 10000 * place);
	}

	bool operator<(const Agent& rhs) const
	{
		return this->time > rhs.time;
	}
};

priority_queue<Agent> que;
set<int> visitedSet;

int main(void)
{
	cin >> N;
	for (int i = 0; i < N; ++i)
	{
		cin >> A[i];
	}

	que.push(Agent(/*time*/0, /*left*/0, /*right*/N - 1, RIGHT));
	que.push(Agent(/*time*/0, /*left*/0, /*right*/N - 1, LEFT));

	while (true)
	{
		Agent agent(que.top());
		que.pop();

		int hash = agent.toInt();
		if (visitedSet.find(hash) != visitedSet.end())
		{
			continue;
		}
		visitedSet.insert(hash);

		if (agent.finished())
		{
			cout << agent.finalTime() << endl;
			return 0;
		}

		que.push(agent.nextRight());
		que.push(agent.nextLeft());
	}
}