If the outer radius of a ring is 2 cm and the inner radius is 1 cm, what is the area of the ring in square centimetres?
一个圆环的外圆半径是2厘米，内圆半径是1厘米，那么圆环的面积是多少平方厘米．
<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>