piece of iron wire just encloses a square with a side length of 4 cm. If this wire is used to enclose a rectangle (with a side length of whole centimeters), what is the maximum area of it?
一根铁丝正好围成边长为4厘米的正方形，如果用这根铁丝围成长方形(边长为整厘米数)，它的面积最大为多少？
<gadget id="calculator">3 * 5</gadget>
<output>15</output>

<result>15</result>