Step 1: Divide the number into separate digits:
1698 => 1 6 9 8
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
1 to 6 = 5
6 to 9 = 3
9 to 8 = 1
8 to 1 = 7
Step 3: Combine the filters to a string:
5 3 1 7 = 5317