Step 1: Divide the number into separate digits:
0981 => 0 9 8 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
0 to 9 = 9
9 to 8 = 1
8 to 1 = 7
1 to 0 = 1
Step 3: Combine the filters to a string:
9 1 7 1 = 9171