Six classes in the sixth grade will have a badminton match, and every two classes will have one match. How many games will there be in total?
六年级6个班要进行羽毛球比赛，每两个班要比赛一场，一共要赛多少场．
<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>