Time Limit: 3000 MS    Memory Limit: 65536 K 


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.


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.


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 ?.

Sample Input

5 ula uka klua kula al 4 jaja baba baja beba 3 marko darko zarko

Sample Output

luka ! ?


coci 2010/2011 contest6