Step 1: Divide the number into separate digits:
9866 => 9 8 6 6
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 8 = 1
8 to 6 = 2
6 to 6 = 0
6 to 9 = 3
Step 3: Combine the filters to a string:
1 2 0 3 = 1203