A 255.2-meter-long fence surrounds a circular vegetable field, and 4 meters are used at the joint. What is the radius of this circular vegetable field?
一段长255.2米的篱笆绕一个圆形菜地围了一圈，接头处用了4米，这个圆形菜地的半径是多少米？
<gadget id="calculator">255.2 - 4</gadget>
<output>251.2</output>

<gadget id="calculator">251.2 / 3.14 / 2</gadget>
<output>40</output>

<result>40</result>