How many four-digit numbers greater than 2000 without repeating numbers can be formed using the four numbers 1, 2, 3, and 4.
用1、2、3、4这四个数字可组成多少个比2000大的无重复数字的四位数．
<gadget id="calculator">6 + 6 + 6</gadget>
<output>18</output>

<result>18</result>