4 children communicate with each other on the phone, and every two people make a call, how many calls will be made in total.
4个小朋友互通电话，每两人通一次话，一共要通多少次电话．
<gadget id="calculator">4 - 1</gadget>
<output>3</output>

<gadget id="calculator">3 / 2</gadget>
<output>3/2 = around 1.5</output>

<gadget id="calculator">4 * (3/2)</gadget>
<output>6</output>

<result>6</result>