There are 6 classes in the second grade in a tug-of-war competition, and there is a competition between every 2 classes. How many games will there be in total?
二年级6个班级进行拔河比赛，每2个班级之间进行一场比赛，一共要进行多少场比赛．
<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>