Palindrome. It is a palindrome.

In this problem, we define a word as a non-empty sequence of characters, denoted as a_1 a_2 \ldots a_n. A palindrome is a word that reads the same forwards and backwards, meaning a_1 a_2 \ldots a_n = a_n a_{n-1} \ldots a_1. If we have two words, S_1 = a_1 a_2 \ldots a_n and S_2 = b_1 b_2 \ldots b_m, then their concatenation is S_1 S_2 = a_1 a_2 \ldots a_n b_1 b_2 \ldots b_m.

You are given a word S_1. Your task is to find the shortest non-empty word S_2 such that the concatenation S_1 S_2 is a palindrome. Note that uppercase and lowercase letters are considered distinct.