5 basketball teams play a game, and every two teams play one game, how many games are there in total?
5个篮球队比赛，每两队要赛一场，一共要赛多少场．
<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>