Step 1: Divide the number into separate digits:
3549 => 3 5 4 9
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
3 to 5 = 2
5 to 4 = 1
4 to 9 = 5
9 to 3 = 6
Step 3: Combine the filters to a string:
2 1 5 6 = 2156