Use steel wire to wind 100 circles of equal size. The radius of the coil is 2.5cm. How long is the steel wire?
用钢丝绕100圈大小相等的圆形，线圈的半径是2.5cm，大约要用多长的钢丝？
<gadget id="calculator">100 / 100</gadget>
<output>1</output>

<gadget id="calculator">2 * 3.14 * 2.5 * 1</gadget>
<output>15.7</output>

<result>15.7</result>