Step 1: Divide the number into separate digits:
9933 => 9 9 3 3
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 9 = 0
9 to 3 = 6
3 to 3 = 0
3 to 9 = 6
Step 3: Combine the filters to a string:
0 6 0 6 = 0606