How many different four-digit numbers are formed by using 0, 6, 7, 8, these 4 numbers?
用0、6、7、8、这4个数字组成不相同的四位数共有多少个．
<gadget id="calculator">3 * 3 * 2 * 1</gadget>
<output>18</output>

<result>18</result>