Couch 2048: How much more till 2048?
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