Step 1: Divide the number into separate digits:
7180 => 7 1 8 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
7 to 1 = 6
1 to 8 = 7
8 to 0 = 8
0 to 7 = 7
Step 3: Combine the filters to a string:
6 7 8 7 = 6787