Step 1: Divide the number into separate digits:
2161 => 2 1 6 1
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 1 = 5
1 to 2 = 1
Step 3: Combine the filters to a string:
1 5 5 1 = 1551