Ivan has got an array of n non-negative integers a1, a2, ..., an. Ivan knows that the array is sorted in the non-decreasing order. 

Ivan wrote out integers 2a1, 2a2, ..., 2an on a piece of paper. Now he wonders, what minimum number of integers of form 2b (b ≥ 0) need to be added to the piece of paper so that the sum of all integers written on the paper equalled 2v - 1 for some integer v (v ≥ 0). 

Help Ivan, find the required quantity of numbers.

Input

The first line contains integer n (1 ≤ n ≤ 105). The second input line contains n space-separated integers a1, a2, ..., an (0 ≤ ai ≤ 2·109). It is guaranteed that a1 ≤ a2 ≤ ... ≤ an.

Output

Print a single integer — the answer to the problem.

Examples

Input

4
0 1 1 1


Output

0


Input

1
3


Output

3

Note

In the first sample you do not need to add anything, the sum of numbers already equals 23 - 1 = 7.

In the second sample you need to add numbers 20, 21, 22.
import java.awt.Rectangle;
import java.awt.geom.Rectangle2D;
import java.io.*;
import java.math.BigInteger;
import java.util.*;

public class C
{
	String line;
	StringTokenizer inputParser;
	BufferedReader is;
	FileInputStream fstream;
	DataInputStream in;
	String FInput="";
	
	
	void openInput(String file)
	{

		if(file==null)is = new BufferedReader(new InputStreamReader(System.in));//stdin
		else
		{
			try{
		
				
			fstream = new FileInputStream(file);
			in = new DataInputStream(fstream);
			is = new BufferedReader(new InputStreamReader(in));
			}catch(Exception e)
			{
				System.err.println(e);
			}
		}

	}
	
	void readNextLine()
	{
		try {
			line = is.readLine();
			inputParser = new StringTokenizer(line, " ");
			//System.err.println("Input: " + line);
		} catch (IOException e) {
			System.err.println("Unexpected IO ERROR: " + e);
		}	
		
	}
	
	int NextInt()
	{
		String n = inputParser.nextToken();
		int val = Integer.parseInt(n);
		
		//System.out.println("I read this number: " + val);
		return val;
	}
	
	String NextString()
	{
		String n = inputParser.nextToken();
		return n;
	}
	
	void closeInput()
	{
		try {
			is.close();
		} catch (IOException e) {
			System.err.println("Unexpected IO ERROR: " + e);
		}
			
	}
	
	public void readFInput()
	{
		for(;;)
		{
			try
			{
				readNextLine();
				FInput+=line+" ";
			}
			catch(Exception e)
			{
				break;
			}
		}
		inputParser = new StringTokenizer(FInput, " ");
	}
	
	long NextLong()
    {
            String n = inputParser.nextToken();
            
            long val = Long.parseLong(n);
            
            return val;
    }
	
	public static void main(String [] argv)
	{
		//String filePath="input.txt";
        String filePath=null;
        if(argv.length>0)filePath=argv[0];
		new C(filePath);
	}

	TreeSet<Integer> ss = new TreeSet<Integer>();
	int MAX=-1;
	public C(String inputFile)
	{
		openInput(inputFile);
		readNextLine();
    	int N=NextInt();
    	readNextLine();
    	int prev=0;
    	int cnt=0;
    	int ret=0;
    	for(int i=0; i<N; i++)
    	{
    		add(NextInt());
    	}
    	ret = MAX - ss.size() + 1;
    	System.out.println(ret);
		closeInput();			
	}
	
	private void add(int x)
	{
		MAX=Math.max(MAX, x);
		if(ss.contains(x))
		{
			ss.remove(x);
			add(x+1);
		}
		else
		ss.add(x);
	}
	
}

