good friends play table tennis. In the preliminary round, every two people play one game. How many games will they play in total?
5个好朋友打乒乓球，初赛时每两个人打一场球，一共打多少场球．
<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>