Xiaogang’s household has a 18-meter fence and a 4-meter-wide rectangular chicken pen. What is its area?
小刚家用18米篱笆靠一面墙围成一个宽4米的长方形养鸡栏，它的面积是多少？
<gadget id="calculator">4 * 2</gadget>
<output>8</output>

<gadget id="calculator">18 - 8</gadget>
<output>10</output>

<gadget id="calculator">10 * 4</gadget>
<output>40</output>

<result>40</result>