Step 1: Divide the number into separate digits:
42507 => 4 2 5 0 7
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
4 to 2 = 2
2 to 5 = 3
5 to 0 = 5
0 to 7 = 7
7 to 4 = 3
Step 3: Combine the filters to a string:
2 3 5 7 3 = 23573