12 teams play a single round-robin game, (every two teams have to play one game), how many games are played in total?
12个球队进行单循环比赛，(每两个队之间都要比赛一场)，一共进行多少场．
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">12 - 1</gadget>
<output>11</output>

<gadget id="calculator">(1/2) * 12 * 11</gadget>
<output>66</output>

<result>66</result>