The radius of the outer circle of a ring is 40 cm and the width of the ring is 10 cm. What is the area of the ring?
一个环形铁皮外圆半径是40厘米，环宽10厘米，这个环形铁皮的面积是多少．
<gadget id="calculator">40 ** 2</gadget>
<output>1_600</output>

<gadget id="calculator">40 - 10</gadget>
<output>30</output>

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

<gadget id="calculator">1_600 - 900</gadget>
<output>700</output>

<gadget id="calculator">3.14 * 700</gadget>
<output>2_198</output>

<result>2_198</result>