From a square iron sheet with a circumference of 40 decimeters, the largest circle is to be cut out. What is the diameter of this circle in decimeters?
从一块周长为40分米的正方形铁皮上，要剪下一个最大的圆，这个圆的直径是多少分米．
<gadget id="calculator">40 / 4</gadget>
<output>10</output>

<result>10</result>