Step 1: Divide the number into separate digits:
67399 => 6 7 3 9 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 7 = 1
7 to 3 = 4
3 to 9 = 6
9 to 9 = 0
9 to 6 = 3
Step 3: Combine the filters to a string:
1 4 6 0 3 = 14603