Six classes have a tug-of-war competition, and every two classes have to play a game. How many games are there in total?
六个班进行拔河比赛，每2个班都要进行一场比赛，共有多少场比赛．
<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>