Step 1: Divide the number into separate digits:
7770 => 7 7 7 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
7 to 7 = 0
7 to 7 = 0
7 to 0 = 7
0 to 7 = 7
Step 3: Combine the filters to a string:
0 0 7 7 = 0077