There are 6 students who communicate with each other on the phone. If every two students make a phone call, how many phone calls are made in total?
有6名同学互通电话，如果每两名同学通一次电话，一共通了多少次电话．
<gadget id="calculator">6 - 1</gadget>
<output>5</output>

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

<gadget id="calculator">5 * 3</gadget>
<output>15</output>

<result>15</result>