import java.io.IOException;
import java.util.*;

import org.omg.CORBA.portable.InputStream;

class Main {
	static int N;
	static int[] A;

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		N = sc.nextInt();
		A = new int[N];
		for (int i = 0; i < N; ++i)
			A[i] = sc.nextInt();
		int[][][] dp = new int[2][N][N];
		for (int k = 0; k < 2; ++k)
			for (int i = 0; i < N; ++i) {
				for (int j = 0; j < N; ++j) {
					dp[k][i][j] = Integer.MAX_VALUE / 2;
				}
			}

		int ans = Integer.MAX_VALUE / 2;
		if (1 <= N - 1) {
			dp[0][1][N - 1] = A[0];
		} else
			ans = Math.min(ans, A[0]);
		if (0 <= N - 2) {
			dp[1][0][N - 2] = A[N - 1];
		} else {
			ans = Math.min(ans, A[N - 1]);
		}
		for (int len = N - 2; len >= 1; --len) {
			for (int left = 0; left < N; ++left) {
				int right = left + len - 1;
				if (right >= N)
					break;
				if (left > 0) {
					dp[0][left][right] = Math.min(dp[1][left - 1][right] + len + 1, dp[0][left - 1][right] + 1);
					dp[0][left][right] = Math.max(dp[0][left][right], A[left - 1]);
				}
				if (right + 1 < N) {
					dp[1][left][right] = Math.min(dp[1][left][right + 1] + 1, dp[0][left][right + 1] + len + 1);
					dp[1][left][right] = Math.max(dp[1][left][right], A[right + 1]);
				}
			}
		}
		for (int i = 0; i < N; ++i) {
			ans = Math.min(ans, Math.max(dp[0][i][i] + 1, A[i]));
			ans = Math.min(ans, Math.max(dp[1][i][i] + 1, A[i]));
		}
		System.out.println(ans);
	}

	static void tr(Object... objects) {
		System.out.println(Arrays.deepToString(objects));
	}

}