Step 1: Divide the number into separate digits:
2451 => 2 4 5 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 4 = 2
4 to 5 = 1
5 to 1 = 4
1 to 2 = 1
Step 3: Combine the filters to a string:
2 1 4 1 = 2141