|Educational Codeforces Round 9|
You're given a list of n strings a 1, a 2, ..., a n. You'd like to concatenate them together in some order such that the resulting string would be lexicographically smallest.
Given the list of strings, output the lexicographically smallest concatenation.
The first line contains integer n — the number of strings (1 ≤ n ≤ 5·104).
Each of the next n lines contains one string a i (1 ≤ |a i| ≤ 50) consisting of only lowercase English letters. The sum of string lengths will not exceed 5·104.
Print the only string a — the lexicographically smallest string concatenation.