11 basketball teams play a single round robin game, with every two teams playing one game. How many games of supply and demand.
11个篮球队进行单循环比赛，每两个队要赛一场．供需比赛多少场．
<gadget id="calculator">11 - 1</gadget>
<output>10</output>

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

<gadget id="calculator">11 * 5</gadget>
<output>55</output>

<result>55</result>