4 good friends call each other to greet each other, and every two people make a phone call, how many times will they have to call in total?
4个好朋友打电话互相问候，每两人通一次电话，一共要通话多少次．
<gadget id="calculator">4 - 1</gadget>
<output>3</output>

<gadget id="calculator">4 / 2</gadget>
<output>2</output>

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

<result>6</result>