Using four pieces of paper with 1, 3, 0, 7 written on it, how many different four-digit numbers can be produced.
用写有1、3、0、7的四张纸片，能排出多少个不同的四位数．
<gadget id="calculator">3 * 3 * 2 * 1</gadget>
<output>18</output>

<result>18</result>