The water lily in a pond doubles every day, until the 18th day the whole pond is covered, and according to this rule, the number of days will grow half of the pond.
一个池塘里的睡莲，每天增长一倍，到第18天长满了整个池塘，按此规律，第多少天长了这个池塘的一半．
<gadget id="calculator">18 - 1</gadget>
<output>17</output>

<result>17</result>