A piece of iron wire is 18 centimeters long, and it encloses a rectangle whose length and width are both full centimeters, what is the maximum area of the rectangle enclosed by it?
一根铁丝长18厘米，把它围成长和宽都是整厘米的长方形，围成长方形的面积最大是多少．
<gadget id="calculator">5 * 4</gadget>
<output>20</output>

<result>20</result>