Step 1: Divide the number into separate digits:
3506 => 3 5 0 6
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
3 to 5 = 2
5 to 0 = 5
0 to 6 = 6
6 to 3 = 3
Step 3: Combine the filters to a string:
2 5 6 3 = 2563