Step 1: Divide the number into separate digits:
8917 => 8 9 1 7
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 7 = 6
7 to 8 = 1
Step 3: Combine the filters to a string:
1 8 6 1 = 1861