Step 1: Divide the number into separate digits:
2815 => 2 8 1 5
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 8 = 6
8 to 1 = 7
1 to 5 = 4
5 to 2 = 3
Step 3: Combine the filters to a string:
6 7 4 3 = 6743