A circular ring has an inner radius of 1 decimeter and an outer radius of 2 decimeters, what is the area of the ring in square decimeters?
一个圆环形，它的内圆半径是1分米，外圆半径是2分米，环形面积是多少平方分米．
<gadget id="calculator">2 ** 2</gadget>
<output>4</output>

<gadget id="calculator">1 ** 2</gadget>
<output>1</output>

<gadget id="calculator">4 - 1</gadget>
<output>3</output>

<gadget id="calculator">3.14 * 3</gadget>
<output>9.42</output>

<result>9.42</result>