Step 1: Divide the number into separate digits:
6875 => 6 8 7 5
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 8 = 2
8 to 7 = 1
7 to 5 = 2
5 to 6 = 1
Step 3: Combine the filters to a string:
2 1 2 1 = 2121