import java.io.BufferedInputStream;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.*;

public class _483 {
	public static void main(String[] args) throws IOException {
		new _483().solve();
	}

	void solve() throws IOException {
		try (final Scanner in = new Scanner(System.in)) {
			int n = in.nextInt();
			A = new int[n];
			for (int i = 0; i < n; i++) {
				A[i] = in.nextInt();
			}
			memo = new int[n][n][2];
			for (int[][] me : memo) for (int[] mo : me) Arrays.fill(mo, -1);
			int ans = INF;
			for (int i = 0; i < n; i++) {
				ans = Math.min(ans, rec(i, i, 0));
			}
			System.out.println(ans);
		}
	}
	
	int[] A;
	final int INF = 1<<30;
	int[][][] memo;
	int rec(int l, int r, int lr) {
		if (l < 0) return INF;
		if (r >= A.length) return INF;
		if (r - l + 1 == A.length) return A[lr == 0 ? l : r];
		if (memo[l][r][lr] != -1) return memo[l][r][lr];
		final int x = lr == 0 ? l : r;
		int ans = INF;
		ans = Math.min(ans, (rec(l - 1, r, 0) + x - l + 1));
		ans = Math.min(ans, (rec(l, r + 1, 1) + r - x + 1));
		ans = Math.max(A[x], ans);
		return memo[l][r][lr] = ans;
	}
	
	// for debug
	static void dump(Object... o) {
		System.err.println(Arrays.deepToString(o));
	}
}