The outer diameter of the ring is 10 cm, the ring width is 1 cm, what is the area of the ring?
圆环的外直径10cm，环宽1cm，圆环的面积多少．
<gadget id="calculator">10 / 2</gadget>
<output>5</output>

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

<gadget id="calculator">3.14 * 25</gadget>
<output>78.5</output>

<gadget id="calculator">5 - 1</gadget>
<output>4</output>

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

<gadget id="calculator">3.14 * 16</gadget>
<output>50.24</output>

<gadget id="calculator">78.5 - 50.24</gadget>
<output>28.26</output>

<result>28.26</result>