import java.util.Scanner;

public class Main {
    static int N=100010;
    static int n;
    static int[] a=new int[N];
    public static void main(String[] args) {
        Scanner in =new Scanner(System.in);
        n=in.nextInt();
        for(int i=1;i<=n;i++){
            a[i]=in.nextInt();
        }
        int l=1;
        int r=100000;
        while (l<r){
            int mid=(l+r)/2;
            if(check_mid(mid)){
                r=mid;
            }else {
                l=mid+1;
            }
        }
        System.out.println(r);
    }

    static boolean check_mid(int e){
        for(int i=1;i<=n;i++){
            e=2*e-a[i];
            if(e<0){
                return false;
            }
            if(e>=100000){
                return true;
            }
        }
        return true;
    }
}