Dongdong used four cards with 0, 1, 2, and 3 to form some four-digit numbers: how many different four-digit numbers can be formed in total.
东东用写有0、1、2、3的四张卡片，组成了一些四位数：一共可以组成多少个不同的四位数．
<gadget id="calculator">3 * 3 * 2 * 1</gadget>
<output>18</output>

<result>18</result>