Couch 2048: How much more till 2048? I have just tried a game called Couch 2048.
(Note: You should have a look to better understand this question.)
Since it wasn't very exciting for me, I've been asking myself 'How much more till 2048!?'
That inspired me to post a challenge, because calculating this in not as easy as I thought.

Your goal:
Given a list of balls on the sofa, you have to output how many balls with a value of 2 have to fall from the sky so that one can win the game (by reaching the 2048 ball).


Assume the input is valid.
Assume the player won't drop any balls.
Assume the balls which fall from the sky always have a value of 2, as I've said.
Valid output examples: 3, "4", [5], ["6"]


Edit:
I should clarify something:
 - You have to print the smallest amount of 2s needed.

Test cases:
[2048] -&gt; 0 You've already won
[1024,1024] -&gt; 0 You don't need any more balls to win
[1024,512,256,128,64,32,16,8,4,2] -&gt; 1 One ball required to 'activate the chain'
[512] -&gt; 768
[512,2] -&gt; 767
[4,16,64] -&gt; 982  

Notes:
I'm not a native speaker - Tell me if you spotted a typo or some non-grammatical text.
If something's unclear, ask in comments.
Java 8, 17 bytes

s-&gt;1024-s.sum()/2

Port of @LuisFelipeDeJesusMunoz' JavaScript answer.
Try it online.
Explanation:
s-&gt;           // Method with IntStream parameter and int return-type
  1024-       //  Return 1024, minus:
   s.sum()    //   The sum of the input-IntStream
          /2  //   Divided by 2