Step 1: Divide the number into separate digits:
2423 => 2 4 2 3
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 4 = 2
4 to 2 = 2
2 to 3 = 1
3 to 2 = 1
Step 3: Combine the filters to a string:
2 2 1 1 = 2211