There is a wall with a length of 7 meters and a width of 5 meters. There is a blackboard on the wall with an area of 4 square meters. Now Master Wang is going to paint this wall. How many square meters is the area to be painted?
有一面墙壁，长是7米，宽是5米．墙上有一块黑板，面积是4平方米．现在王师傅要粉刷这面墙壁，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>