Step 1: Divide the number into separate digits:
1871 => 1 8 7 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
1 to 8 = 7
8 to 7 = 1
7 to 1 = 6
1 to 1 = 0
Step 3: Combine the filters to a string:
7 1 6 0 = 7160