Step 1: Divide the number into separate digits:
3613 => 3 6 1 3
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
3 to 6 = 3
6 to 1 = 5
1 to 3 = 2
3 to 3 = 0
Step 3: Combine the filters to a string:
3 5 2 0 = 3520