Step 1: Divide the number into separate digits:
7477 => 7 4 7 7
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
7 to 4 = 3
4 to 7 = 3
7 to 7 = 0
7 to 7 = 0
Step 3: Combine the filters to a string:
3 3 0 0 = 3300