Step 1: Divide the number into separate digits:
9855 => 9 8 5 5
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 8 = 1
8 to 5 = 3
5 to 5 = 0
5 to 9 = 4
Step 3: Combine the filters to a string:
1 3 0 4 = 1304