Given a string `s` which consists of lowercase or uppercase letters, return the length of the longest palindrome that can be built with those letters.

Letters are case sensitive, for example, `"Aa"` is not considered a palindrome here.

Example 1:

    Input: s = "abccccdd"
    Output: 7
    Explanation:
    One longest palindrome that can be built is "dccaccd", whose length is 7.

Example 2:

    Input: s = "a"
    Output: 1

Example 3:

    Input: s = "bb"
    Output: 2

Constraints:

- `1 <= s.length <= 2000`
- `s` consists of lowercase and/or uppercase English letters only.