20 female students from class 3 and 1 participated in the singing competition and dance competition in the children's palace. Among them, 13 people participated in the singing competition and 15 people participated in the dancing competition. How many people participated in both?
三1班的20个女同学参加了少年宫的歌咏比赛和舞蹈比赛．其中有13人参加了歌咏比赛，15人参加了舞蹈比赛．这两项都参加的有多少人？
<gadget id="calculator">15 - 20</gadget>
<output>-5</output>

<gadget id="calculator">13 + (-5)</gadget>
<output>8</output>

<result>8</result>