Step 1: Divide the number into separate digits:
5606 => 5 6 0 6
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
5 to 6 = 1
6 to 0 = 6
0 to 6 = 6
6 to 5 = 1
Step 3: Combine the filters to a string:
1 6 6 1 = 1661