Step 1: Divide the number into separate digits:
0931 => 0 9 3 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
0 to 9 = 9
9 to 3 = 6
3 to 1 = 2
1 to 0 = 1
Step 3: Combine the filters to a string:
9 6 2 1 = 9621