A circular part has an outer diameter of 10 decimeters and an inner radius of 4 decimeters. What is the area of the ring part.
一个环形零件，外圆直径是10分米，内圆半径4分米．环形零件的面积是多少．
<gadget id="calculator">10 / 2</gadget>
<output>5</output>

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

<gadget id="calculator">3.14 * 25</gadget>
<output>78.5</output>

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

<gadget id="calculator">3.14 * 16</gadget>
<output>50.24</output>

<gadget id="calculator">78.5 - 50.24</gadget>
<output>28.26</output>

<result>28.26</result>