Step 1: Divide the number into separate digits:
7797 => 7 7 9 7
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
7 to 7 = 0
7 to 9 = 2
9 to 7 = 2
7 to 7 = 0
Step 3: Combine the filters to a string:
0 2 2 0 = 0220