Step 1: Divide the number into separate digits:
6525 => 6 5 2 5
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
6 to 5 = 1
5 to 2 = 3
2 to 5 = 3
5 to 6 = 1
Step 3: Combine the filters to a string:
1 3 3 1 = 1331