There are 20 telephone poles on the road, and there is a billboard between every two poles. How many billboards are there in total?
公路上有20根电线杆，每两根电线杆之间有一块广告牌，一共有多少块广告牌．
<gadget id="calculator">20 - 1</gadget>
<output>19</output>

<gadget id="calculator">1 * 19</gadget>
<output>19</output>

<result>19</result>