Teacher Yang wants to make a cuboid book box without a cover for the class, which is 20 cm long, 14 cm wide, and 21 cm high. How many square centimeters of cardboard should be used to make this book box?
杨老师要为班级做一个无盖长方体书箱，长20厘米，宽14厘米，高21厘米，做这个书箱至少用多少平方厘米的纸板？
<gadget id="calculator">20 * 14</gadget>
<output>280</output>

<gadget id="calculator">20 * 21</gadget>
<output>420</output>

<gadget id="calculator">14 * 21</gadget>
<output>294</output>

<gadget id="calculator">280 + 420 + 294</gadget>
<output>994</output>

<gadget id="calculator">994 * 2</gadget>
<output>1_988</output>

<gadget id="calculator">1_988 - 280</gadget>
<output>1_708</output>

<result>1_708</result>