Step 1: Divide the number into separate digits:
91813 => 9 1 8 1 3
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 1 = 7
1 to 3 = 2
3 to 9 = 6
Step 3: Combine the filters to a string:
8 7 7 2 6 = 87726