Step 1: Divide the number into separate digits:
5721 => 5 7 2 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
5 to 7 = 2
7 to 2 = 5
2 to 1 = 1
1 to 5 = 4
Step 3: Combine the filters to a string:
2 5 1 4 = 2514