A caterpillar grows from a larva to an adult, doubles in size every day, and grows to 20 mm in 18 days. How many days does it take to grow to 10 mm?
一条毛毛虫从幼虫长成成虫，每天长1倍，18天长到20毫米，当长到10毫米时需用多少天．
<result>17</result>