Step 1: Divide the number into separate digits:
21917 => 2 1 9 1 7
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 1 = 1
1 to 9 = 8
9 to 1 = 8
1 to 7 = 6
7 to 2 = 5
Step 3: Combine the filters to a string:
1 8 8 6 5 = 18865