classmates write letters to each other, each writes a letter to the others, how many letters are written in total?
3个同学互相写信，每人都给其他人写一封，一共写了多少封信．
<gadget id="calculator">3 - 1</gadget>
<output>2</output>

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

<result>6</result>