Step 1: Divide the number into separate digits:
18398 => 1 8 3 9 8
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
1 to 8 = 7
8 to 3 = 5
3 to 9 = 6
9 to 8 = 1
8 to 1 = 7
Step 3: Combine the filters to a string:
7 5 6 1 7 = 75617