The wall in front of the classroom is 8 meters long and 4 meters wide. There is a blackboard on the wall with an area of 4 square meters. If you want to paint this wall now, how many square meters will you have to paint?
教室前面的墙壁，长8米，宽4米．墙上有一块黑板，面积是4平方米．如果现在要粉刷这面墙壁，要粉刷的面积是多少平方米？
<gadget id="calculator">8 * 4</gadget>
<output>32</output>

<gadget id="calculator">32 - 4</gadget>
<output>28</output>

<result>28</result>