Step 1: Divide the number into separate digits:
8911 => 8 9 1 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
8 to 9 = 1
9 to 1 = 8
1 to 1 = 0
1 to 8 = 7
Step 3: Combine the filters to a string:
1 8 0 7 = 1807