There are teams from 4 countries participating in a football match with the Chinese team. There is a match between every 2 teams. How many matches are there in total?
有4个国家的球队和中国队参加足球赛，每2支球队之间都进行一场比赛，共赛多少场．
<gadget id="calculator">4 / 2</gadget>
<output>2</output>

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

<result>10</result>