Step 1: Divide the number into separate digits:
0903 => 0 9 0 3
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
0 to 9 = 9
9 to 0 = 9
0 to 3 = 3
3 to 0 = 3
Step 3: Combine the filters to a string:
9 9 3 3 = 9933