Step 1: Divide the number into separate digits:
3805 => 3 8 0 5
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
3 to 8 = 5
8 to 0 = 8
0 to 5 = 5
5 to 3 = 2
Step 3: Combine the filters to a string:
5 8 5 2 = 5852