Step 1: Divide the number into separate digits:
6915 => 6 9 1 5
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 9 = 3
9 to 1 = 8
1 to 5 = 4
5 to 6 = 1
Step 3: Combine the filters to a string:
3 8 4 1 = 3841