Close to a wall, a rectangular vegetable field is surrounded by an 18-meter-long bamboo fence. What is the maximum area of this vegetable field in square meters (length and width are integers).
紧靠一道围墙边，用18米长的竹篱笆围出一块长方形的菜地，这块菜地的面积最大是多少平方米(长和宽是整数)．
<result>40</result>