Two 3s, one 1 and one 2 can be used to form different four-digit numbers. How many of these four-digit numbers are there?
用两个3、一个1和一个2可组成不同的四位数，这些四位数共有多少个．
<gadget id="calculator">3 + 3 + 6</gadget>
<output>12</output>

<result>12</result>