Step 1: Divide the number into separate digits:
18408 => 1 8 4 0 8
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
1 to 8 = 7
8 to 4 = 4
4 to 0 = 4
0 to 8 = 8
8 to 1 = 7
Step 3: Combine the filters to a string:
7 4 4 8 7 = 74487