The wall in front of the classroom is 7 meters long and 5 meters wide, and there is a blackboard with an area of 4 square meters on the wall. Now I want to paint this wall, how many square meters is the area to be painted?
教室前面的墙长7米、宽5米，墙上有一块面积为4平方米的黑板．现在要粉刷这面墙，问要粉刷的面积是多少平方米？
<gadget id="calculator">7 * 5</gadget>
<output>35</output>

<gadget id="calculator">35 - 4</gadget>
<output>31</output>

<result>31</result>