5 youth football teams, every two teams have to play one game, how many games will there be in total?
5支少年足球队，每两支球队都要比赛一场，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>