In a basketball game, 6 teams play in a round robin, how many games do they need to play?
在一次篮球比赛中，6个队进行循环赛，需要比赛多少场．
<gadget id="calculator">6 - 1</gadget>
<output>5</output>

<gadget id="calculator">6 / 2</gadget>
<output>3</output>

<gadget id="calculator">5 * 3</gadget>
<output>15</output>

<result>15</result>