The program print the count of characters which are repeated in next n characters. Help me to solve this

Question

Input
abcddcba 5

Output: 6 (repeated)

Explanation:
The character a at index 0 is not
repeated within the next 5 characters or
previous 5 characters.
The character b at index 1 is repeated
within the next 5 characters.
The character c at index 2 is repeated
within the next 5 characters.
The character d at index 3 is repeated
within the next 5 characters.
The character d at index 4 is repeated
within the previous 5 characters.
The character cat index 5 is repeated
within the previous 5 characters.
The character b at index 6 is repeated
within the previous 5 characters.
The character a at index 7 is not
repeated within the next 5 characters
or previous 5 characters.

Answer 1

Then? What is the problem?