Step 1: Divide the number into separate digits:
81491 => 8 1 4 9 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
8 to 1 = 7
1 to 4 = 3
4 to 9 = 5
9 to 1 = 8
1 to 8 = 7
Step 3: Combine the filters to a string:
7 3 5 8 7 = 73587