The school held a table tennis and badminton competition. There were 12 people from Class 3 and 3 participating, 9 people participated in the table tennis competition, and 7 people participated in the badminton competition. How many people participated in both.
学校举行乒乓球和羽毛球大赛，三3班共有12人参赛，有9人参加乒乓球比赛，有7人参加羽毛球比赛，两项都参加的有多少人．
<gadget id="calculator">9 - 12</gadget>
<output>-3</output>

<gadget id="calculator">7 + (-3)</gadget>
<output>4</output>

<result>4</result>