The wall in front of the classroom is 6 meters long and 3 meters wide. There is a blackboard on the wall, which is 3 meters long and 1 meter wide. Now to paint this wall, 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，长是3米，宽是1米．现在要粉刷这面墙壁，要粉刷的面积是多少平方米？
<gadget id="calculator">6 * 3</gadget>
<output>18</output>

<gadget id="calculator">3 * 1</gadget>
<output>3</output>

<gadget id="calculator">18 - 3</gadget>
<output>15</output>

<result>15</result>