Step 1: Divide the number into separate digits:
61805 => 6 1 8 0 5
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 1 = 5
1 to 8 = 7
8 to 0 = 8
0 to 5 = 5
5 to 6 = 1
Step 3: Combine the filters to a string:
5 7 8 5 1 = 57851