Zhang Dong’s family bought a cabinet-type air conditioner. Its outer packaging is a cuboid carton with a length of 0.5 meters, a width of 0.4 meters, and a height of 1.8 meters. How many square meters of cardboard is needed to make such a carton? (neglected at joints)
张冬家买了一台柜式空调，它的外包装是一个长0.5米、宽0.4米、高1.8米的长方体纸箱．做这样一个纸箱至少需要硬纸板多少平方米？(接头处忽略不计)
<gadget id="calculator">0.5 * 0.4</gadget>
<output>0.2</output>

<gadget id="calculator">0.5 * 1.8</gadget>
<output>0.9</output>

<gadget id="calculator">0.4 * 1.8</gadget>
<output>0.72</output>

<gadget id="calculator">0.2 + 0.9 + 0.72</gadget>
<output>1.82</output>

<gadget id="calculator">1.82 * 2</gadget>
<output>3.64</output>

<result>3.64</result>