There are 20 trees planted on one side of a straight path. If you want to hang a signboard on 3 adjacent trees, how many different ways are there to hang the signboard?
一条笔直的小路一边栽了20棵树，要想在其中相邻的3棵树上挂上一个标记牌，一共有多少种不同的挂标记牌的方法．
<gadget id="calculator">20 - 2</gadget>
<output>18</output>

<result>18</result>