There are 18 willow trees on one side of the park, and there is a billboard between every two willow trees. How many billboards are there in total?
公园的一侧有18棵柳树，每两棵柳树之间有一个广告牌，一共有多少个广告牌．
<gadget id="calculator">18 - 1</gadget>
<output>17</output>

<result>17</result>