A circular iron plate has an inner radius of 3 decimeters and a ring width of 1 decimeter. What is the area of the circular iron plate in square decimeters?
一个环形铁片，内圆半径是3分米，环宽1分米，这个环形铁片的面积是多少平方分米．
<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<gadget id="calculator">4 ** 2</gadget>
<output>16</output>

<gadget id="calculator">3 ** 2</gadget>
<output>9</output>

<gadget id="calculator">16 - 9</gadget>
<output>7</output>

<gadget id="calculator">3.14 * 7</gadget>
<output>21.98</output>

<result>21.98</result>