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();
			}
			int ans = Integer.MAX_VALUE;
			for (int i = 0; i < n; i++) {
				if (ok(i, A[i], -1, -1) && ok(i + 1, A[i] - 2 * i - 1, 1, -1)) {
					ans = Math.min(ans, A[i]);
				}
				if (ok(i, A[i], 1, -1) && ok(i - 1, A[i] - 2 * (n - 1 - i) - 1, -1, -1)) {
					ans = Math.min(ans, A[i]);
				}
				if (ok(0, A[i] - i, 1, 1)) {
					ans = Math.min(ans, A[i] + (n - 1 - i));
				}
				if (ok(0, A[i] + i, 1, -1)) {
					ans = Math.min(ans, A[i] + i);
				}
			}
			System.out.println(ans);
		}
	}
	
	int[] A;
	boolean ok(int x, int y, int dx, int dy) {
		for (; x >= 0 && x < A.length; x += dx, y += dy) {
			if (y < A[x]) {
				return false;
			}
		}
		return true;
	}
	
	// for debug
	static void dump(Object... o) {
		System.err.println(Arrays.deepToString(o));
	}
}