If every two people make one phone call, how many times do 4 people need to call in total?
每两人通一次电话，4人一共需要通话多少次．
<gadget id="calculator">4 / 2</gadget>
<output>2</output>

<gadget id="calculator">3 * 2</gadget>
<output>6</output>

<result>6</result>