#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 <cstring>
#include <algorithm>

int N;
int A[2000];

int dp[2000][2000][2];
const int INF = 1e9;

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

	memset(dp, 0x3f, sizeof dp);
	dp[0][N - 1][0] = A[0];
	dp[0][N - 1][1] = A[N - 1];

	for (int i = 0; i < N; ++i)
	{
		for (int j = N - 1; j >= 0; --j)
		{
			if (i > j)
			{
				continue;
			}

			if (i - 1 >= 0)
			{
				dp[i][j][0] = min(dp[i][j][0], max(A[i], dp[i - 1][j][0] + 1));
				dp[i][j][1] = min(dp[i][j][0], max(A[j], dp[i - 1][j][0] + j - (i - 1)));
			}
			if (j + 1 < N)
			{
				dp[i][j][0] = min(dp[i][j][0], max(A[i], dp[i][j + 1][1] + (j + 1) - i));
				dp[i][j][1] = min(dp[i][j][1], max(A[j], dp[i][j + 1][1] + 1));
			}
		}
	}

	int ans = INF;
	for (int i = 0; i < N; ++i)
	{
		ans = min(ans, dp[i][i][0]);
	}

	cout << ans << endl;
}