Step 1: Divide the number into separate digits:
21763 => 2 1 7 6 3
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 1 = 1
1 to 7 = 6
7 to 6 = 1
6 to 3 = 3
3 to 2 = 1
Step 3: Combine the filters to a string:
1 6 1 3 1 = 16131