To make 100 iron rings with a radius of 20 cm, at least how many meters of iron wire are needed.
做半径是20厘米的铁环100个，至少需要铁丝多少米．
<gadget id="calculator">100 / 100</gadget>
<output>1</output>

<gadget id="calculator">3.14 * 2 * 20 * 1</gadget>
<output>125.6</output>

<result>125.6</result>