Step 1: Divide the number into separate digits:
1891 => 1 8 9 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
1 to 8 = 7
8 to 9 = 1
9 to 1 = 8
1 to 1 = 0
Step 3: Combine the filters to a string:
7 1 8 0 = 7180