There are two piles of stones on the construction site, the first pile of stones weighs 4.5 tons, and the second pile is 0.9 tons less than twice the weight of the first pile. How many tons does the second pile of stones weigh?
工地有两堆石子，第一堆石子重4.5吨，第二堆比第一堆的2倍少0.9吨．第二堆石子重多少吨？
<gadget id="calculator">9 - 4.5</gadget>
<output>4.5</output>

<result>4.5</result>