If the radius of the inner circle is 4 cm and the width of the ring is 1 cm, what is the area of the ring in square centimeters?
一个环形，如果内圆的半径是4厘米，环形的宽度是1厘米，那么环形的面积是多少平方厘米．
<gadget id="calculator">4 + 1</gadget>
<output>5</output>

<gadget id="calculator">5 ** 2</gadget>
<output>25</output>

<gadget id="calculator">4 ** 2</gadget>
<output>16</output>

<gadget id="calculator">25 - 16</gadget>
<output>9</output>

<gadget id="calculator">3.14 * 9</gadget>
<output>28.26</output>

<result>28.26</result>