Step 1: Divide the number into separate digits:
9777 => 9 7 7 7
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
9 to 7 = 2
7 to 7 = 0
7 to 7 = 0
7 to 9 = 2
Step 3: Combine the filters to a string:
2 0 0 2 = 2002