Step 1: Divide the number into separate digits:
1670 => 1 6 7 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
1 to 6 = 5
6 to 7 = 1
7 to 0 = 7
0 to 1 = 1
Step 3: Combine the filters to a string:
5 1 7 1 = 5171