There are 42 students in Class 4: 31 students who subscribe to "Youth Daily", and 27 students who subscribe to "Fairy Tale". Everyone subscribes to at least one of the newspapers, and how many students subscribe to both "Youth Daily" and "Fairy Tale Newspaper".
四1班有42名同学：订阅《少年报》的有31名同学，订阅《童话报》的有27名同学．每人至少订其中一份报纸，有多少名同学既订《少年报》又订《童话报》．
<gadget id="calculator">31 - 42</gadget>
<output>-11</output>

<gadget id="calculator">27 + (-11)</gadget>
<output>16</output>

<result>16</result>