Step 1: Divide the number into separate digits:
5577 => 5 5 7 7
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
5 to 5 = 0
5 to 7 = 2
7 to 7 = 0
7 to 5 = 2
Step 3: Combine the filters to a string:
0 2 0 2 = 0202