In a group of children, 12 people have watched the cartoon "Black Cat Sheriff", 21 people have watched the cartoon "Havoc in Heaven", and 8 people have watched both cartoons. May I ask: How many children have seen at least one of them?
在一群小朋友中，有12人看过动画片《黑猫警长》，有21人看过动画片《大闹天宫》，并且有8人两部动画片都看过．请问：至少看过其中一部的小朋友有多少人？
<gadget id="calculator">21 - 8</gadget>
<output>13</output>

<gadget id="calculator">12 + 13</gadget>
<output>25</output>

<result>25</result>