Step 1: Divide the number into separate digits:
1902 => 1 9 0 2
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
1 to 9 = 8
9 to 0 = 9
0 to 2 = 2
2 to 1 = 1
Step 3: Combine the filters to a string:
8 9 2 1 = 8921