Step 1: Divide the number into separate digits:
7554 => 7 5 5 4
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
7 to 5 = 2
5 to 5 = 0
5 to 4 = 1
4 to 7 = 3
Step 3: Combine the filters to a string:
2 0 1 3 = 2013