Step 1: Divide the number into separate digits:
2165 => 2 1 6 5
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 1 = 1
1 to 6 = 5
6 to 5 = 1
5 to 2 = 3
Step 3: Combine the filters to a string:
1 5 1 3 = 1513