Step 1: Divide the number into separate digits:
6536 => 6 5 3 6
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 5 = 1
5 to 3 = 2
3 to 6 = 3
6 to 6 = 0
Step 3: Combine the filters to a string:
1 2 3 0 = 1230