Using the four numbers 0, 1, 2, and 3, how many different four-digit numbers can be formed in total?
用0、1、2、3这4个数，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>