Step 1: Divide the number into separate digits:
2587 => 2 5 8 7
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 5 = 3
5 to 8 = 3
8 to 7 = 1
7 to 2 = 5
Step 3: Combine the filters to a string:
3 3 1 5 = 3315