Step 1: Divide the number into separate digits:
8615 => 8 6 1 5
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
8 to 6 = 2
6 to 1 = 5
1 to 5 = 4
5 to 8 = 3
Step 3: Combine the filters to a string:
2 5 4 3 = 2543