6 football teams are playing a game, and every 2 teams play one game, how many games will be played in total.
6个足球队进行比赛，每2个球队踢一场，一共要踢多少场．
<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>