Step 1: Divide the number into separate digits:
15518 => 1 5 5 1 8
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
1 to 5 = 4
5 to 5 = 0
5 to 1 = 4
1 to 8 = 7
8 to 1 = 7
Step 3: Combine the filters to a string:
4 0 4 7 7 = 40477