A wall is 9 meters long and 3 meters high, and there is a blackboard with an area of 4 square meters in the middle. Now this wall is to be painted, what is the painted area?
一面墙长9米，高3米，中间有一块面积为4平方米的黑板，现在要粉刷这面墙，粉刷的面积是多少？
<gadget id="calculator">9 * 3</gadget>
<output>27</output>

<gadget id="calculator">27 - 4</gadget>
<output>23</output>

<result>23</result>