There are 6 people who send each other a greeting card, how many greeting cards have they sent in total?
有6个人，他们互寄一张贺卡，一共寄了多少张贺卡．
<gadget id="calculator">6 - 1</gadget>
<output>5</output>

<gadget id="calculator">5 * 6</gadget>
<output>30</output>

<result>30</result>