6174 is known as Kaprekar's constant after the Indian mathematician D. R. Kaprekar. This number is notable for the following rule:

- Take any four-digit number, using at least two different digits (leading zeros are allowed).

- Arrange the digits in descending and then in ascending order to get two four-digit numbers, adding leading zeros if necessary.

- Subtract the smaller number from the bigger number.

- Go back to step 2 and repeat.

The above process, known as Kaprekar's routine, will always reach its fixed point, 6174, in at most 7 iterations. Once 6174 is reached, the process will continue yielding 7641 1467 = 6174. For example, choose 1495:

9541 1459 = 8082

8820 0288 = 8532

8532 2358 = 6174

7641 1467 = 6174