5 football teams compete. There is one game between each two teams, how many games will be played in total.
5支足球队进行比赛．每两队之间比赛一场，一共要比赛多少场．
<gadget id="calculator">5 - 1</gadget>
<output>4</output>

<gadget id="calculator">5 / 2</gadget>
<output>5/2 = around 2.5</output>

<gadget id="calculator">4 * (5/2)</gadget>
<output>10</output>

<result>10</result>