Step 1: Divide the number into separate digits:
2400 => 2 4 0 0
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 4 = 2
4 to 0 = 4
0 to 0 = 0
0 to 2 = 2
Step 3: Combine the filters to a string:
2 4 0 2 = 2402