There are 42 students in Class 1 of the third grade, including 39 students in the art class, 34 students in the gymnastics class, 30 students in the swimming class, and 37 students in the Mathematical Olympiad class. So, at least how many students in this class participate in all four classes.
三年级一班有学生42人，其中参加美术班的同学有39人，参加体操班的同学有34人，参加游泳班的同学有30人，参加奥数班的同学有37人．那么，这个班至少有多少个学生这四种班都参加．
<gadget id="calculator">39 - 42</gadget>
<output>-3</output>

<gadget id="calculator">34 + (-3)</gadget>
<output>31</output>

<gadget id="calculator">30 - 42</gadget>
<output>-12</output>

<gadget id="calculator">31 + (-12)</gadget>
<output>19</output>

<gadget id="calculator">37 - 42</gadget>
<output>-5</output>

<gadget id="calculator">19 + (-5)</gadget>
<output>14</output>

<result>14</result>