A kind of aquatic plant grows twice as much as the previous day every day. On the 20th day, it just covers the entire pond, so how many days does it fill up half of the pond?
一种水草，每天比前一天增长1倍，第20天刚好长满全部水池，那么第多少天正好长满水池的一半．
<gadget id="calculator">20 - 1</gadget>
<output>19</output>

<result>19</result>