Step 1: Divide the number into separate digits:
15728 => 1 5 7 2 8
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
1 to 5 = 4
5 to 7 = 2
7 to 2 = 5
2 to 8 = 6
8 to 1 = 7
Step 3: Combine the filters to a string:
4 2 5 6 7 = 42567