There are 5 friends in different cities, if they call each other once, how many calls will they have to make in total?
有5个朋友在不同的城市，如果他们之间互通一次电话，那么共要通话多少次．
<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>