Use 24 small black or white squares to form a large rectangle. It is known that all black squares are used on the periphery of the formed rectangle, so how many black squares are there at least?
用24个黑色或白色的小正方形拼成一个大长方形，已知拼成的长方形外围用的都是黑色正方形，那么黑色正方形至少有多少个．
<gadget id="calculator">6 + 4</gadget>
<output>10</output>

<gadget id="calculator">10 * 2</gadget>
<output>20</output>

<gadget id="calculator">20 - 4</gadget>
<output>16</output>

<result>16</result>