5 teams play a single round robin game (every two teams have to play one game), how many games will be played in total.
5个球队进行单循环比赛(每两队之间都要比赛一场)，一共要进行多少场比赛．
<gadget id="calculator">5 - 1</gadget>
<output>4</output>

<gadget id="calculator">4 / 2</gadget>
<output>2</output>

<gadget id="calculator">5 * 2</gadget>
<output>10</output>

<result>10</result>