Step 1: Divide the number into separate digits:
64517 => 6 4 5 1 7
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 4 = 2
4 to 5 = 1
5 to 1 = 4
1 to 7 = 6
7 to 6 = 1
Step 3: Combine the filters to a string:
2 1 4 6 1 = 21461