There are 10 trees planted on both sides of the central avenue of the school, and there is a flower bed between every two trees. How many flower beds are there in total?
学校中心大道两边各栽了10棵树，每两棵树间有1个花坛，一共有多少个花坛．
<gadget id="calculator">10 - 1</gadget>
<output>9</output>

<gadget id="calculator">9 * 2</gadget>
<output>18</output>

<result>18</result>