Step 1: Divide the number into separate digits:
7452 => 7 4 5 2
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
7 to 4 = 3
4 to 5 = 1
5 to 2 = 3
2 to 7 = 5
Step 3: Combine the filters to a string:
3 1 3 5 = 3135