Step 1: Divide the number into separate digits:
3515 => 3 5 1 5
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
3 to 5 = 2
5 to 1 = 4
1 to 5 = 4
5 to 3 = 2
Step 3: Combine the filters to a string:
2 4 4 2 = 2442