Step 1: Divide the number into separate digits:
15902 => 1 5 9 0 2
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
1 to 5 = 4
5 to 9 = 4
9 to 0 = 9
0 to 2 = 2
2 to 1 = 1
Step 3: Combine the filters to a string:
4 4 9 2 1 = 44921