Sum palindrome-change costs over all substrings

You are given a DNA string `dna` consisting only of characters `A`, `C`, `G`, `T`. For any substring `dna[l..r]`, define its **palindrome modification cost** as the minimum number of single-character substitutions required to make that substring a palindrome. (Equivalently, it is the number of mismatched character pairs when comparing from both ends: count of indices `i` where `dna[l+i] != dna[r-i]` for `i < (r-l+1)/2`.) Compute the **sum of palindrome modification costs over all substrings** of `dna`, and output the result as a 64-bit integer. ### Input - A single string `dna`. ### Output - A single integer: the sum of costs across all substrings. ### Notes - You may assume the result fits in a signed 64-bit integer (`long` / `int64`).