Step 1: Divide the number into separate digits:
1815 => 1 8 1 5
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
1 to 8 = 7
8 to 1 = 7
1 to 5 = 4
5 to 1 = 4
Step 3: Combine the filters to a string:
7 7 4 4 = 7744