The area covered by water lilies in a pond doubles every day, and the entire pond is just covered in 20 days. How many days will it take to cover only half of the pond?
某池中的睡莲所遮盖的面积，每天扩大1倍，20天恰好遮住整个水池，问若只遮住水池的一半需要多少天．
<result>19</result>