Using the four numbers 0, 1, 9, and 8, how many four-digit numbers can be formed without repetition.
用0、1、9、8四个数字，可以组成多少个没有重复的四位数．
<gadget id="calculator">3 * 3 * 2 * 1</gadget>
<output>18</output>

<result>18</result>