Given two strings `a` and `b`, return the length of the longest uncommon subsequence between `a` and `b`. If the longest uncommon subsequence does not exist, return `-1`.

An uncommon subsequence between two strings is a string that is a subsequence of one but not the other.

A subsequence of a string `s` is a string that can be obtained after deleting any number of characters from `s`.

- For example, `"abc"` is a subsequence of `"aebdc"` because you can delete the underlined characters in `"aebdc"` to get `"abc"`. Other subsequences of `"aebdc"` include `"aebdc"`, `"aeb"`, and `""` (empty string).

Example 1:

    Input: a = "aba", b = "cdc"
    Output: 3
    Explanation: One longest uncommon subsequence is "aba" because "aba" is a subsequence of "aba" but not "cdc".
    Note that "cdc" is also a longest uncommon subsequence.

Example 2:

    Input: a = "aaa", b = "bbb"
    Output: 3
    Explanation: The longest uncommon subsequences are "aaa" and "bbb".

Example 3:

    Input: a = "aaa", b = "aaa"
    Output: -1
    Explanation: Every subsequence of string a is also a subsequence of string b. Similarly, every subsequence of string b is also a subsequence of string a.

Constraints:

- `1 <= a.length, b.length <= 100`
- `a` and `b` consist of lower-case English letters.