Step 1: Divide the number into separate digits:
1850 => 1 8 5 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
1 to 8 = 7
8 to 5 = 3
5 to 0 = 5
0 to 1 = 1
Step 3: Combine the filters to a string:
7 3 5 1 = 7351