Step 1: Divide the number into separate digits:
6606 => 6 6 0 6
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 6 = 0
6 to 0 = 6
0 to 6 = 6
6 to 6 = 0
Step 3: Combine the filters to a string:
0 6 6 0 = 0660