The wall in front of the classroom is 6 meters long and 3 meters wide. There is a blackboard on the wall with an area of 4 square meters. Now this wall is going to be painted, how many square meters is the area to be painted?
教室前面的墙壁长6米，宽3米．墙上有一块黑板，面积是4平方米．现在要粉刷这面壁，要粉刷的面积是多少平方米？
<gadget id="calculator">6 * 3</gadget>
<output>18</output>

<gadget id="calculator">18 - 4</gadget>
<output>14</output>

<result>14</result>