Step 1: Divide the number into separate digits:
0777 => 0 7 7 7
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
0 to 7 = 7
7 to 7 = 0
7 to 7 = 0
7 to 0 = 7
Step 3: Combine the filters to a string:
7 0 0 7 = 7007