Step 1: Divide the number into separate digits:
7152 => 7 1 5 2
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
7 to 1 = 6
1 to 5 = 4
5 to 2 = 3
2 to 7 = 5
Step 3: Combine the filters to a string:
6 4 3 5 = 6435