Step 1: Divide the number into separate digits:
9805 => 9 8 0 5
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 8 = 1
8 to 0 = 8
0 to 5 = 5
5 to 9 = 4
Step 3: Combine the filters to a string:
1 8 5 4 = 1854