Step 1: Divide the number into separate digits:
2503 => 2 5 0 3
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 5 = 3
5 to 0 = 5
0 to 3 = 3
3 to 2 = 1
Step 3: Combine the filters to a string:
3 5 3 1 = 3531