A cylindrical oil barrel is a square when its side is expanded. It is known that the radius of the bottom of the oil barrel is 5 decimeters. What is the height of the oil barrel in decimeters?
一个圆柱形油桶，侧面展开是一个正方形，已知这个油桶的底面半径是5分米，那么油桶的高是多少分米．
<gadget id="calculator">2 * 3.14 * 5</gadget>
<output>31.4</output>

<result>31.4</result>