As we know,Rikka is poor at math. Yuta is worrying about this situation,so he gives Rikka some math tasks to practice. There is one of them:


One day,Yuta got a string which contains n letters but Rikka lost it in accident. Now they want to recover the string. Yuta remembers that the string only contains lowercase letters and it is not a palindrome string. Unfortunately he cannot remember some letters. Can you help him recover the string?


It is too difficult for Rikka. Can you help her?

This problem has multi test cases (no more than 20). For each test case,The first line contains a number n(1≤n≤1000). The next line contains an n-length string which only contains lowercase letters and ‘?’

（编辑：李大同）

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