The outer diameter of the ring is 10cm, the width of the ring is 2cm, what is the area of the ring?
圆环外直径10cm，环宽2cm，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 - 2</gadget>
<output>3</output>

<gadget id="calculator">3 ** 2</gadget>
<output>9</output>

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

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

<result>50.24</result>