문제

Author: Luka Kalinovčić

A list of words written in some unknown alphabet was found. It is known, however, that these words are in alphabetic order. Write a program that will find the unique alphabetic ordering of used letters, or determine that no such ordering exists or that there is more than one possible solution.

입력

The first line of input contains a positive integer N (N ≤ 100), the number of words. The following N lines contain the list of words found, one word per line. Each word consists of at most 10 lowercase letters.

출력

The first and only line of output should contain all letters in alphabetic order. If no such ordering exists, output ‘!’. If there is more than one solution, output ‘?’.

예제 입력 1

5
ula
uka
klua
kula
al

예제 출력 1

luka

예제 입력 2

4
jaja
baba
baja
beba

예제 출력 2

!

예제 입력 3

3
marko
darko
zarko

예제 출력 3

?