20 teams play a single round-robin game, and every two teams have to play once, how many games will there be in total?
20支球队进行单循环比赛，每两个队要赛一次，则一共要比赛多少场．
<gadget id="calculator">20 - 1</gadget>
<output>19</output>

<gadget id="calculator">19 / 2</gadget>
<output>19/2 = around 9.5</output>

<gadget id="calculator">20 * (19/2)</gadget>
<output>190</output>

<result>190</result>