Step 1: Divide the number into separate digits:
8957 => 8 9 5 7
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
8 to 9 = 1
9 to 5 = 4
5 to 7 = 2
7 to 8 = 1
Step 3: Combine the filters to a string:
1 4 2 1 = 1421