The diameter of the outer circle of a ring is 100cm, and the radius of the inner circle is 4dm. What is the area of the ring in dm**2.
一个圆环的外圆直径是100cm，内圆半径是4dm，这个圆环的面积是多少dm**2．
<gadget id="calculator">100 / 10 / 2</gadget>
<output>5</output>

<gadget id="calculator">5 ** 2</gadget>
<output>25</output>

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

<gadget id="calculator">25 - 16</gadget>
<output>9</output>

<gadget id="calculator">3.14 * 9</gadget>
<output>28.26</output>

<result>28.26</result>