Step 1: Divide the number into separate digits:
9800 => 9 8 0 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 8 = 1
8 to 0 = 8
0 to 0 = 0
0 to 9 = 9
Step 3: Combine the filters to a string:
1 8 0 9 = 1809