A circular piece of iron has an outer diameter of 0.4m and a ring width of 0.1m. What is its area?
一块环形铁片，外直径0.4m，环宽0.1m，它的面积是多少？
<gadget id="calculator">0.4 / 2</gadget>
<output>0.2</output>

<gadget id="calculator">0.2 ** 2</gadget>
<output>0.04</output>

<gadget id="calculator">3.14 * 0.04</gadget>
<output>0.1256</output>

<gadget id="calculator">0.2 - 0.1</gadget>
<output>0.1</output>

<gadget id="calculator">0.1 ** 2</gadget>
<output>0.01</output>

<gadget id="calculator">3.14 * 0.01</gadget>
<output>0.0314</output>

<gadget id="calculator">0.1256 - 0.0314</gadget>
<output>0.0942</output>

<result>0.0942</result>