Step 1: Divide the number into separate digits:
3908 => 3 9 0 8
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
3 to 9 = 6
9 to 0 = 9
0 to 8 = 8
8 to 3 = 5
Step 3: Combine the filters to a string:
6 9 8 5 = 6985