teams play a single round robin match (every two teams play a game), how many games will there be in total?
6个球队进行单循环赛(每两个球队之间进行一场比赛)，一共要比赛多少场．
<gadget id="calculator">6 - 1</gadget>
<output>5</output>

<gadget id="calculator">5 / 2</gadget>
<output>5/2 = around 2.5</output>

<gadget id="calculator">6 * (5/2)</gadget>
<output>15</output>

<result>15</result>