There are 4 children, they send each other a greeting card, how many greeting cards have they sent in total?
有4个小朋友，他们互寄一张贺卡，一共寄了多少张贺卡．
<gadget id="calculator">4 - 1</gadget>
<output>3</output>

<gadget id="calculator">3 * 4</gadget>
<output>12</output>

<result>12</result>