Step 1: Divide the number into separate digits:
9798 => 9 7 9 8
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 7 = 2
7 to 9 = 2
9 to 8 = 1
8 to 9 = 1
Step 3: Combine the filters to a string:
2 2 1 1 = 2211