A ring with an outer diameter of 8 decimeters. The radius of the inner circle is 2 decimeters, what is the area of the circle in square decimeters?
一个环形，外圆直径是8分米．内圆半径是2分米，这个环形的面积是多少平方分米．
<gadget id="calculator">8 / 2</gadget>
<output>4</output>

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

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

<gadget id="calculator">16 - 4</gadget>
<output>12</output>

<gadget id="calculator">3.14 * 12</gadget>
<output>37.68</output>

<result>37.68</result>