In order to welcome the World Cup, 5 classes in the fourth grade hold a football match, and every two classes have to play one game, so how many games will they have in total?
为了迎接世界杯，四年级5个班举行足球比赛，每两个班级都要赛一场，那么他们一共要比赛多少场．
<gadget id="calculator">5 - 1</gadget>
<output>4</output>

<gadget id="calculator">4 / 2</gadget>
<output>2</output>

<gadget id="calculator">5 * 2</gadget>
<output>10</output>

<result>10</result>