Step 1: Divide the number into separate digits:
9877 => 9 8 7 7
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 8 = 1
8 to 7 = 1
7 to 7 = 0
7 to 9 = 2
Step 3: Combine the filters to a string:
1 1 0 2 = 1102