There are 20 street lights on one side of the road, and there is a billboard between every two street lights. How many billboards are there in total?
马路的一边有20盏路灯，每两盏路灯中间有一个广告牌．一共有多少个广告牌．
<gadget id="calculator">20 - 1</gadget>
<output>19</output>

<result>19</result>