Step 1: Divide the number into separate digits:
7504 => 7 5 0 4
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
7 to 5 = 2
5 to 0 = 5
0 to 4 = 4
4 to 7 = 3
Step 3: Combine the filters to a string:
2 5 4 3 = 2543