Step 1: Divide the number into separate digits:
91992 => 9 1 9 9 2
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 1 = 8
1 to 9 = 8
9 to 9 = 0
9 to 2 = 7
2 to 9 = 7
Step 3: Combine the filters to a string:
8 8 0 7 7 = 88077