There is a water lily in a pond, which doubles in size every day, and can completely cover the pond after 20 days. So, how many days does it take for water lilies to cover half of the pond?
有一个池塘中的睡莲，每天长大一倍，经过20天可以把池塘全部遮满．那么，睡莲要遮住半个池塘需要经过多少天？
<gadget id="calculator">20 - 1</gadget>
<output>19</output>

<result>19</result>