6174 is known as Kaprekar's constant. This number is notable for the following property:
1. Take any four-digit number, using at least two different digits. (Leading zeros are allowed.)
2. Arrange the digits in ascending and then in descending order to get two four-digit numbers, adding leading zeros if necessary.
3. Subtract the smaller number from the bigger number.
4. Go back to step 2
The above process will always reach 6174 in at most 7 iterations.