A pond needs to be planted with water lilies. The water lilies double in size every day. It is known that in 30 days, the whole pond can be filled, and how many days can half of the pond be covered.
一个池塘要种睡莲，睡莲每天成一倍生长，已知30天能长满全池，多少天能长满半池．
<gadget id="calculator">30 - 1</gadget>
<output>29</output>

<result>29</result>