The school playground is approximately rectangular with a length of 120 meters and a width of 50 meters. A green belt is planted around it. How long is the green belt?
学校操场是一个长120米、宽50米的近似长方形，在它的四周种上绿化带，绿化带长多少米？
<gadget id="calculator">120 + 50</gadget>
<output>170</output>

<gadget id="calculator">170 * 2</gadget>
<output>340</output>

<result>340</result>