Uncle Li surrounded a rectangular horse circle with 20 1-meter-long fences. The length of the enclosed horse pen is 9 meters, what is the area in square meters?
李大爷用20根1米长的栅栏围一个长方形马圈．围成的马圈长9米，面积是多少平方米．
<gadget id="calculator">20 / 2</gadget>
<output>10</output>

<gadget id="calculator">10 - 9</gadget>
<output>1</output>

<gadget id="calculator">9 * 1</gadget>
<output>9</output>

<result>9</result>