Step 1: Divide the number into separate digits:
8703 => 8 7 0 3
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
8 to 7 = 1
7 to 0 = 7
0 to 3 = 3
3 to 8 = 5
Step 3: Combine the filters to a string:
1 7 3 5 = 1735