Step 1: Divide the number into separate digits:
8715 => 8 7 1 5
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
8 to 7 = 1
7 to 1 = 6
1 to 5 = 4
5 to 8 = 3
Step 3: Combine the filters to a string:
1 6 4 3 = 1643