Use 18 cm long iron wire to form various rectangles, and the length and width are required to be integer centimeters. What is the maximum area of the enclosed rectangle?
用18厘米长的铁丝围成各种长方形，要求长和宽的长度都是整厘米数．围成的长方形的面积最大是多少？
<gadget id="calculator">5 * 4</gadget>
<output>20</output>

<result>20</result>