Step 1: Divide the number into separate digits:
5340 => 5 3 4 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
5 to 3 = 2
3 to 4 = 1
4 to 0 = 4
0 to 5 = 5
Step 3: Combine the filters to a string:
2 1 4 5 = 2145