Step 1: Divide the number into separate digits:
9721 => 9 7 2 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 7 = 2
7 to 2 = 5
2 to 1 = 1
1 to 9 = 8
Step 3: Combine the filters to a string:
2 5 1 8 = 2518