Step 1: Divide the number into separate digits:
2977 => 2 9 7 7
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
2 to 9 = 7
9 to 7 = 2
7 to 7 = 0
7 to 2 = 5
Step 3: Combine the filters to a string:
7 2 0 5 = 7205