There is a circular goldfish pond with a diameter of 15 meters in the children's park. To make 4 circles of circular railings around the goldfish pond, how many steel bars should be used at least?
儿童公园有一个直径是15米的圆形金鱼池，在金鱼池周围要做4圈圆形栏杆，至少要用多少钢条？
<gadget id="calculator">3.14 * 15 * 4</gadget>
<output>188.4</output>

<result>188.4</result>