Step 1: Divide the number into separate digits:
7637 => 7 6 3 7
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
7 to 6 = 1
6 to 3 = 3
3 to 7 = 4
7 to 7 = 0
Step 3: Combine the filters to a string:
1 3 4 0 = 1340