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();
		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));
	}

	static class Scanner {
		private final java.io.InputStream in = System.in;
		private final byte[] buffer = new byte[1024];
		private int ptr = 0;
		private int buflen = 0;

		private boolean hasNextByte() {
			if (ptr < buflen) {
				return true;
			} else {
				ptr = 0;
				try {
					buflen = in.read(buffer);
				} catch (IOException e) {
					e.printStackTrace();
				}
				if (buflen <= 0) {
					return false;
				}
			}
			return true;
		}

		private int readByte() {
			if (hasNextByte())
				return buffer[ptr++];
			else
				return -1;
		}

		private boolean isPrintableChar(int c) {
			return 33 <= c && c <= 126;
		}

		public boolean hasNext() {
			while (hasNextByte() && !isPrintableChar(buffer[ptr]))
				ptr++;
			return hasNextByte();
		}

		public String next() {
			if (!hasNext())
				throw new NoSuchElementException();
			StringBuilder sb = new StringBuilder();
			int b = readByte();
			while (isPrintableChar(b)) {
				sb.appendCodePoint(b);
				b = readByte();
			}
			return sb.toString();
		}

		public long nextLong() {
			if (!hasNext())
				throw new NoSuchElementException();
			long n = 0;
			boolean minus = false;
			int b = readByte();
			if (b == '-') {
				minus = true;
				b = readByte();
			}
			if (b < '0' || '9' < b) {
				throw new NumberFormatException();
			}
			while (true) {
				if ('0' <= b && b <= '9') {
					n *= 10;
					n += b - '0';
				} else if (b == -1 || !isPrintableChar(b)) {
					return minus ? -n : n;
				} else {
					throw new NumberFormatException();
				}
				b = readByte();
			}
		}

		public int nextInt() {
			long nl = nextLong();
			if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE)
				throw new NumberFormatException();
			return (int) nl;
		}

		public double nextDouble() {
			return Double.parseDouble(next());
		}
	}

}