A circular piece of iron has an inner radius of 20 cm and a width of 10 cm. What is the area of the ring in square centimeters?
一个环形铁片，内圆半径20厘米，圆环宽10厘米，这环铁片面积是多少平方厘米？
<gadget id="calculator">20 + 10</gadget>
<output>30</output>

<gadget id="calculator">30 ** 2</gadget>
<output>900</output>

<gadget id="calculator">20 ** 2</gadget>
<output>400</output>

<gadget id="calculator">900 - 400</gadget>
<output>500</output>

<gadget id="calculator">3.14 * 500</gadget>
<output>1_570</output>

<result>1_570</result>