Step 1: Divide the number into separate digits:
2355 => 2 3 5 5
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 3 = 1
3 to 5 = 2
5 to 5 = 0
5 to 2 = 3
Step 3: Combine the filters to a string:
1 2 0 3 = 1203