Step 1: Divide the number into separate digits:
31090 => 3 1 0 9 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
3 to 1 = 2
1 to 0 = 1
0 to 9 = 9
9 to 0 = 9
0 to 3 = 3
Step 3: Combine the filters to a string:
2 1 9 9 3 = 21993