Xiao Ming is going to make a book cover, which is 20 cm long, 14 cm wide, and 21 cm high. What is the minimum square centimeter of cardboard needed to make such a book? (the book cover does not have the right side)
小明要做一个书套，长20厘米，宽14厘米，高21厘米．做这样一个书至少要用多少平方厘米的硬纸板？(书套没有右面)
<gadget id="calculator">20 * 14</gadget>
<output>280</output>

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

<gadget id="calculator">280 + 420</gadget>
<output>700</output>

<gadget id="calculator">700 * 2</gadget>
<output>1_400</output>

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

<gadget id="calculator">1_400 + 294</gadget>
<output>1_694</output>

<result>1_694</result>