To make a metal ventilation pipe with a radius of 15 cm and a length of 140 cm, how many square centimeters of iron should be used? (neglected at joints)
做一个半径为15厘米、长140厘米的铁皮通风管，至少要用多少平方厘米铁皮？(接头处忽略不计)
<gadget id="calculator">2 * 3.14 * 15 * 140</gadget>
<output>13_188</output>

<result>13_188</result>