Step 1: Divide the number into separate digits:
9780 => 9 7 8 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 7 = 2
7 to 8 = 1
8 to 0 = 8
0 to 9 = 9
Step 3: Combine the filters to a string:
2 1 8 9 = 2189