Use 18 cm long iron wire to form various rectangles. The length and width are required to be integer centimeters. What is the maximum area of the enclosed rectangle in square centimeters?
用长18厘米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平方厘米．
<gadget id="calculator">5 * 4</gadget>
<output>20</output>

<result>20</result>