Step 1: Divide the number into separate digits:
21440 => 2 1 4 4 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 1 = 1
1 to 4 = 3
4 to 4 = 0
4 to 0 = 4
0 to 2 = 2
Step 3: Combine the filters to a string:
1 3 0 4 2 = 13042