Step 1: Divide the number into separate digits:
20402 => 2 0 4 0 2
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 0 = 2
0 to 4 = 4
4 to 0 = 4
0 to 2 = 2
2 to 2 = 0
Step 3: Combine the filters to a string:
2 4 4 2 0 = 24420