Step 1: Divide the number into separate digits:
0870 => 0 8 7 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
0 to 8 = 8
8 to 7 = 1
7 to 0 = 7
0 to 0 = 0
Step 3: Combine the filters to a string:
8 1 7 0 = 8170