Step 1: Divide the number into separate digits:
1940 => 1 9 4 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
1 to 9 = 8
9 to 4 = 5
4 to 0 = 4
0 to 1 = 1
Step 3: Combine the filters to a string:
8 5 4 1 = 8541