There are at most 4 digits, and any two of them have at least two digits different.
最多有多少个4位数，它们中的任意两个至少有两位的数字不同．
<gadget id="calculator">1_000 / 10</gadget>
<output>100</output>

<gadget id="calculator">100 * 9</gadget>
<output>900</output>

<result>900</result>