Step 1: Divide the number into separate digits:
1536 => 1 5 3 6
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
1 to 5 = 4
5 to 3 = 2
3 to 6 = 3
6 to 1 = 5
Step 3: Combine the filters to a string:
4 2 3 5 = 4235