Step 1: Divide the number into separate digits:
64375 => 6 4 3 7 5
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 4 = 2
4 to 3 = 1
3 to 7 = 4
7 to 5 = 2
5 to 6 = 1
Step 3: Combine the filters to a string:
2 1 4 2 1 = 21421