Step 1: Divide the number into separate digits:
1638 => 1 6 3 8
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
1 to 6 = 5
6 to 3 = 3
3 to 8 = 5
8 to 1 = 7
Step 3: Combine the filters to a string:
5 3 5 7 = 5357