Step 1: Divide the number into separate digits:
7872 => 7 8 7 2
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
7 to 8 = 1
8 to 7 = 1
7 to 2 = 5
2 to 7 = 5
Step 3: Combine the filters to a string:
1 1 5 5 = 1155