Step 1: Divide the number into separate digits:
1155 => 1 1 5 5
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
1 to 1 = 0
1 to 5 = 4
5 to 5 = 0
5 to 1 = 4
Step 3: Combine the filters to a string:
0 4 0 4 = 0404