import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

//由题目可知,无论能量大与小,都满足 e = 2 * e - h[i];
//初始能量越大,最终的结果越大,要找到一个满足条件的最小值
//可以根据二分的向左找模板:
/*

    if(check(mid)) r = mid;
	else l = mid + 1;
			
*/

public class Main{
	static int n;
	static int N = 100010;
	static int[] g = new int[N];
	static BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
	public static boolean check(int e) {
		for(int i = 1;i <= n;i++) {
			e = 2 * e - g[i];
			if(e > 1e5) return true;
			if(e < 0) return false;
		}
		return true;
	}
	public static void main(String[] args) throws IOException{
		n = Integer.parseInt(in.readLine());
		String[] data = in.readLine().split(" ");
		for(int i = 1;i <= n;i ++)  g[i] = Integer.parseInt(data[i - 1]);
	
		
		int l = 0, r = 100000;
		while(l < r) {
			int mid = l + r >> 1;
			if(check(mid)) r = mid;
			else {
				l = mid + 1;
			}
		}
		System.out.println(l);
		in.close();
	}
	
}