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