Grandpa bought 20 bottles of beer, and every 2 empty bottles can be exchanged for one bottle of beer, so how many bottles of beer can grandpa drink at most?
爷爷买了20瓶啤酒，每2个空瓶子能换回一瓶啤酒，那么爷爷最多能喝上多少瓶啤酒．
<gadget id="calculator">20 + 10 + 5 + 2 + 1 + 1 + 1</gadget>
<output>40</output>

<result>40</result>