Step 1: Divide the number into separate digits:
9077 => 9 0 7 7
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 0 = 9
0 to 7 = 7
7 to 7 = 0
7 to 9 = 2
Step 3: Combine the filters to a string:
9 7 0 2 = 9702