You are given an array of strings `arr`. A string `s` is formed by the concatenation of a subsequence of `arr` that has unique characters.

Return the maximum possible length of `s`.

A subsequence is an array that can be derived from another array by deleting some or no elements without changing the order of the remaining elements.

Example 1:

    Input: arr = ["un","iq","ue"]
    Output: 4
    Explanation: All the valid concatenations are:
    - ""
    - "un"
    - "iq"
    - "ue"
    - "uniq" ("un" + "iq")
    - "ique" ("iq" + "ue")
    Maximum length is 4.

Example 2:

    Input: arr = ["cha","r","act","ers"]
    Output: 6
    Explanation: Possible longest valid concatenations are "chaers" ("cha" + "ers") and "acters" ("act" + "ers").

Example 3:

    Input: arr = ["abcdefghijklmnopqrstuvwxyz"]
    Output: 26
    Explanation: The only string in arr has all 26 characters.

Constraints:

- `1 <= arr.length <= 16`
- `1 <= arr[i].length <= 26`
- `arr[i]` contains only lowercase English letters.