Step 1: Divide the number into separate digits:
1351 => 1 3 5 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
1 to 3 = 2
3 to 5 = 2
5 to 1 = 4
1 to 1 = 0
Step 3: Combine the filters to a string:
2 2 4 0 = 2240