Step 1: Divide the number into separate digits:
2438 => 2 4 3 8
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 4 = 2
4 to 3 = 1
3 to 8 = 5
8 to 2 = 6
Step 3: Combine the filters to a string:
2 1 5 6 = 2156