kind of grass is planted in a pond, and every day the grass becomes twice the grass of the previous day, until the 10th day, the grass is just full of the pond, and the grass in the pond is half of the pond on the number of days.
一个池塘中种下一种草，每过一天草就变为前一天草的2倍，到第10天刚好草长满池塘，第多少天池塘中草为池塘的一半．
<gadget id="calculator">10 - 1</gadget>
<output>9</output>

<result>9</result>