Step 1: Divide the number into separate digits:
8900 => 8 9 0 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
8 to 9 = 1
9 to 0 = 9
0 to 0 = 0
0 to 8 = 8
Step 3: Combine the filters to a string:
1 9 0 8 = 1908