import java.io.*;

public class Main {
    public static void main(String[] args) throws IOException {
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
        int n = Integer.parseInt(reader.readLine().trim());
        String[] temp = reader.readLine().split(" ");
        int[] h = new int[n];
        for (int i = 0; i < n; i++) {
            h[i] = Integer.parseInt(temp[i]);
        }
        int l = 0, r = 100000;
        while(l < r){
            int mid = (l + r) >> 1;
            int e = mid;
            for (int i = 0; i < n && e >= 0 && e <= 100000; i++) {
                e = 2*e - h[i];
            }
            if (e < 0) l = mid + 1;
            else r = mid;
        }
        System.out.println(l);
    }
}