1. Pick a 4 digit numbers with at least 2 distinct number
   ex: 1337
2. Rearrange the digits to form largest and smallest number
   7331 & 1337
3. Subtract largest to smallest
   7331 - 1337 = 5994
4. Repeat step 2
   9954 - 4599 = 5355
5. You will eventually get 6174 in less than 7 repetitions
6. Why stop at 6174? because 7641 - 1467 = 6174

TEST