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();}}