Step 1: Divide the number into separate digits:
39618 => 3 9 6 1 8
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
3 to 9 = 6
9 to 6 = 3
6 to 1 = 5
1 to 8 = 7
8 to 3 = 5
Step 3: Combine the filters to a string:
6 3 5 7 5 = 63575