Step 1: Divide the number into separate digits:
20309 => 2 0 3 0 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 0 = 2
0 to 3 = 3
3 to 0 = 3
0 to 9 = 9
9 to 2 = 7
Step 3: Combine the filters to a string:
2 3 3 9 7 = 23397