Step 1: Divide the number into separate digits:
1339 => 1 3 3 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
1 to 3 = 2
3 to 3 = 0
3 to 9 = 6
9 to 1 = 8
Step 3: Combine the filters to a string:
2 0 6 8 = 2068