You are given two strings `s` and `t`.

String `t` is generated by random shuffling string `s` and then add one more letter at a random position.

Return the letter that was added to `t`.

Example 1:

    Input: s = "abcd", t = "abcde"
    Output: "e"
    Explanation: 'e' is the letter that was added.

Example 2:

    Input: s = "", t = "y"
    Output: "y"

Constraints:

- `0 <= s.length <= 1000`
- `t.length == s.length + 1`
- `s` and `t` consist of lowercase English letters.