The wall in front of the classroom is 9 meters long and 5 meters wide. There is a blackboard on the wall. The area is 4 square meters. If this wall is to be painted, what is the area to be painted?
教室前面的墙壁，长9米，宽5米，墙上有一块黑板，面积是4平方米，要粉刷这面墙，要粉刷的面积是多少？
<gadget id="calculator">9 * 5</gadget>
<output>45</output>

<gadget id="calculator">45 - 4</gadget>
<output>41</output>

<result>41</result>