worm grows from a larva to an adult, doubles its size every day, grows to 20 cm in 20 days, and how many days does it take to grow to 5 cm.
一只小虫从幼虫长到成虫，每天长前一天的一倍，20天长到20厘米，长到5厘米时用了多少天．
<gadget id="calculator">20 - 1 - 1</gadget>
<output>18</output>

<result>18</result>