Step 1: Divide the number into separate digits:
7851 => 7 8 5 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
7 to 8 = 1
8 to 5 = 3
5 to 1 = 4
1 to 7 = 6
Step 3: Combine the filters to a string:
1 3 4 6 = 1346