A 4dm long iron wire forms a circle with 0.1dm remaining, what is the circumference of this circle?
一根长4dm的铁丝围成一个圆还剩0.1dm，这个圆的周长是多少．
<gadget id="calculator">4 - 0.1</gadget>
<output>3.9</output>

<result>3.9</result>