Step 1: Divide the number into separate digits:
6876 => 6 8 7 6
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 6 = 1
6 to 6 = 0
Step 3: Combine the filters to a string:
2 1 1 0 = 2110