Step 1: Divide the number into separate digits:
7591 => 7 5 9 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
7 to 5 = 2
5 to 9 = 4
9 to 1 = 8
1 to 7 = 6
Step 3: Combine the filters to a string:
2 4 8 6 = 2486