How many different four-digit numbers can be formed by using the four numbers 1, 2, 3, and 0 to form a four-digit number.
用1、2、3、0四个数字组成四位数，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>