There is a roll of paper, the diameter of the reel is 4 cm, the outer diameter of the roll is 10 cm, and the thickness of each layer of paper is 0.5 mm, find the length of the roll.
有一卷筒纸，卷轴直径为4厘米，卷纸外直径为10厘米，每层纸的厚度为0.5mm，求这卷纸的长度．
<gadget id="calculator">10 / 2</gadget>
<output>5</output>

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

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

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

<gadget id="calculator">25 - 4</gadget>
<output>21</output>

<gadget id="calculator">0.5 / 10</gadget>
<output>0.05</output>

<gadget id="calculator">21 / 0.05</gadget>
<output>420</output>

<gadget id="calculator">3.14 * 420</gadget>
<output>1_318.8</output>

<result>1_318.8</result>