A total of 19 people from class 31 participated in the singing and dancing competition, among which 12 people participated in singing, 11 people participated in dancing, and how many people participated in both.
三1班共有19人参加歌舞大赛，其中参加歌唱的有12人，参加跳舞的有11人，两项都参加的有多少人．
<gadget id="calculator">11 - 19</gadget>
<output>-8</output>

<gadget id="calculator">12 + (-8)</gadget>
<output>4</output>

<result>4</result>