Step 1: Divide the number into separate digits:
6688 => 6 6 8 8
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 6 = 0
6 to 8 = 2
8 to 8 = 0
8 to 6 = 2
Step 3: Combine the filters to a string:
0 2 0 2 = 0202