using System.Collections.Generic;
using System;

public class Hello
{
    static void Main()
    {
        var n = int.Parse(Console.ReadLine().Trim());
        getAns(n);
    }
    static string getT(string s, char c)
    {
        s += c;
        var s2 = s.ToCharArray();
        Array.Sort(s2);
        return new string(s2);
    }
    static void getAns(int n)
    {
        var d = new Dictionary<string, int>();
        for (int i = 0; i < n; i++)
        {
            var s = Console.ReadLine().Trim();
            for (int j = 0; j < 26; j++)
            {
                var t = getT(s, (char)(j + 'a'));
                if (d.ContainsKey(t)) d[t]++;
                else d[t] = 1;
            }
        }
        foreach (var x in d)
            if (x.Value == 1) { Console.WriteLine(x.Key); return; }
        Console.WriteLine(-1);
    }
}