import java.util.*;

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

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		N = sc.nextInt();
		A = new long[N];
		for (int i = 0; i < N; ++i)
			A[i] = sc.nextLong();
		long[][][] dp = new long[N][N][2];
		for (int i = 0; i < N; ++i) {
			for (int j = 0; j < N; ++j) {
				for (int k = 0; k < 2; ++k)
					dp[i][j][k] = Long.MAX_VALUE / 10;
			}
		}
		// 0左
		// 1右
		dp[1][N - 1][0] = A[0];
		dp[0][N - 2][1] = A[N - 1];
		for (int len = N - 2; len >= 1; --len) {
			for (int left = 0; left < N; ++left) {
				int right = left + len - 1;
				if (left > 0) {
					dp[left][right][0] = Math.min(dp[left - 1][right][1] + len + 1, dp[left - 1][right][0] + 1);
					dp[left][right][0] = Math.max(dp[left][right][0], A[left - 1]);
				}
				if (right + 1 < N) {
					dp[left][right][1] = Math.min(dp[left][right + 1][1] + 1, dp[left][right + 1][0] + len + 1);
					dp[left][right][1] = Math.max(dp[left][right][1], A[right + 1]);
				}
			}
		}
		long ans = Long.MAX_VALUE / 10;
		for (int i = 0; i < N; ++i) {
			ans = Math.min(ans, Math.max(dp[i][i][0] + 1, A[i]));
			ans = Math.min(ans, Math.max(dp[i][i][1] + 1, A[i]));
		}
		System.out.println(ans);
	}

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

}