Gorkimov, a writer of the former Soviet Union, once found the number 6 174 wonderful, and listed it as an "unsolved mystery" in the book Sensitivity of Mathematics. Now please write a four-digit number casually, but the four digits can't be exactly the same, such as 6666 and 9999, but the three digits are the same.

After writing this number, arrange it in the following order: rearrange it from largest to smallest, raise the largest number to thousands, and so on, such as 5477, and it will become 7754 after arrangement.

Next, reverse the obtained number, and then find the difference between the two numbers, and subtract the small one from the big one, only looking at the absolute value, not the symbol. Then, continue to sort out the difference according to the above method, and finally get another difference. Repeat this several times, and 6 174 will appear. This seems to be a "black hole" in mathematics. Any four digits that are not exactly the same will fall into this "black hole" after the above "rearrangement" and "difference".

The specific calculation process of the above example is as follows:

( 1)7754-4577=3 177(2)773 1- 1377=6354(3)6543-3456=3087

(4)8730-0378=8532(5)8532-2358=6 174

Note that numbers starting with 0 here should also be regarded as four digits.