Problem:
Score : 100 points

Problem Statement

Kode Festival is an anual contest where the hardest stone in the world is determined. (Kode is a Japanese word for "hardness".)

This year, 2^N stones participated. The hardness of the i-th stone is A_i.

In the contest, stones are thrown at each other in a knockout tournament.

When two stones with hardness X and Y are thrown at each other, the following will happen:

When X > Y:
The stone with hardness Y will be destroyed and eliminated.
The hardness of the stone with hardness X will become X-Y.

When X = Y:
One of the stones will be destroyed and eliminated.
The hardness of the other stone will remain the same.

When X < Y:
The stone with hardness X will be destroyed and eliminated.
The hardness of the stone with hardness Y will become Y-X.

The 2^N stones will fight in a knockout tournament as follows:

The following pairs will fight: (the 1-st stone versus the 2-nd stone), (the 3-rd stone versus the 4-th stone), ...

The following pairs will fight: (the winner of (1-st versus 2-nd) versus the winner of (3-rd versus 4-th)), (the winner of (5-th versus 6-th) versus the winner of (7-th versus 8-th)), ...

And so forth, until there is only one stone remaining.

Determine the eventual hardness of the last stone remaining.

Constraints

1 \leq N \leq 18

1 \leq A_i \leq 10^9

A_i is an integer.

Input

The input is given from Standard Input in the following format:

N
A_1
A_2
:
A_{2^N}

Output

Print the eventual hardness of the last stone remaining.

Sample Input 1

2
1
3
10
19

Sample Output 1

7

Sample Input 2

3
1
3
2
4
6
8
100
104

Sample Output 2

2

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System.Collections.Generic;
using System.Linq;
using System;

public class Hello
{
    public static void Main()
    {
        var a = new List<int>();
        var n = int.Parse(Console.ReadLine().Trim());
        var imax = (int)Math.Pow(2, n - 1);
        for (int i = 0; i < imax; i++)
        {
            var w = int.Parse(Console.ReadLine().Trim());
            var w2 = int.Parse(Console.ReadLine().Trim());
            if (w == w2) a.Add(w);
            else a.Add(Math.Abs(w - w2));
        }
        var ret = getAns(a);
        Console.WriteLine(ret[0]);
    }
    public static  List<int>  getAns (List<int> w)
    {
        var wc = w.Count();
        if (wc == 1) return w;
        var ret = new List<int>();
        for (int i = 0; i < wc/2; i++)
        {
            var t = w[2 * i];
            var t2 = w[2 * i + 1];
            if (t == t2) ret.Add(t);
            else ret.Add(Math.Abs(t - t2));
        }
        return getAns(ret);
    }
}