Step 1: Divide the number into separate digits:
2689 => 2 6 8 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 6 = 4
6 to 8 = 2
8 to 9 = 1
9 to 2 = 7
Step 3: Combine the filters to a string:
4 2 1 7 = 4217