Step 1: Divide the number into separate digits:
91877 => 9 1 8 7 7
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 1 = 8
1 to 8 = 7
8 to 7 = 1
7 to 7 = 0
7 to 9 = 2
Step 3: Combine the filters to a string:
8 7 1 0 2 = 87102