Four classmates agreed during the holiday that each pair of students should exchange a letter, how many letters should they write in total?
4个同学在假期里约定，每两人互通一封信，他们共要写多少封信．
<gadget id="calculator">4 - 1</gadget>
<output>3</output>

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

<result>12</result>