A professional chicken farmer uses a 15-meter-long fence to surround a rectangular chicken farm with one wall, and the length and width of the rectangle are integers. What is the maximum area of the chicken farm?
一个养鸡专业户用一段长15米的篱笆靠一面墙围成的一个长方形养鸡场，并且长方形长和宽为整数，养鸡场的面积最大有多大？
<gadget id="calculator">4 * 2</gadget>
<output>8</output>

<gadget id="calculator">15 - 8</gadget>
<output>7</output>

<gadget id="calculator">7 * 4</gadget>
<output>28</output>

<result>28</result>