Step 1: Divide the number into separate digits:
2507 => 2 5 0 7
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 5 = 3
5 to 0 = 5
0 to 7 = 7
7 to 2 = 5
Step 3: Combine the filters to a string:
3 5 7 5 = 3575