You are given two strings s and t. String t is generated by randomly shuffling string s and then adding one more letter at a random position. Return the letter that was added to t.
find_the_difference("abcd", "abcde") ➞ "e"
find_the_difference("", "y") ➞ "y"
find_the_difference("ae", "aea") ➞ "a"N/A