To install guardrails around a pool that is 9 meters long and 6 meters wide, how many meters are needed for the guardrails?
要给一个长9米，宽6米的水池四周装上护栏，那么护栏需要多少米．
<gadget id="calculator">9 + 6</gadget>
<output>15</output>

<gadget id="calculator">15 * 2</gadget>
<output>30</output>

<result>30</result>