How many different four-digit numbers can be formed by using the four number cards 0, 1, 4, and 7?
用0、1、4、7这四张数字卡片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>