Step 1: Divide the number into separate digits:
30900 => 3 0 9 0 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
3 to 0 = 3
0 to 9 = 9
9 to 0 = 9
0 to 0 = 0
0 to 3 = 3
Step 3: Combine the filters to a string:
3 9 9 0 3 = 39903