Step 1: Divide the number into separate digits:
2015 => 2 0 1 5
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 0 = 2
0 to 1 = 1
1 to 5 = 4
5 to 2 = 3
Step 3: Combine the filters to a string:
2 1 4 3 = 2143