Step 1: Divide the number into separate digits:
20171 => 2 0 1 7 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 0 = 2
0 to 1 = 1
1 to 7 = 6
7 to 1 = 6
1 to 2 = 1
Step 3: Combine the filters to a string:
2 1 6 6 1 = 21661