using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;

class Program
{
    static int NN => int.Parse(ReadLine());
    static int[] NList => ReadLine().Split().Select(int.Parse).ToArray();
    static int[][] NArr(long n) => Enumerable.Repeat(0, (int)n).Select(_ => NList).ToArray();
    static string[] SList(long n) => Enumerable.Repeat(0, (int)n).Select(_ => ReadLine()).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var n = NN;
        var s = SList(n);
        var dic = new Dictionary<Data, List<int>>();
        for (var i = 0; i < n; ++i)
        {
            var counts = new int[26];
            foreach (var c in s[i]) ++counts[c - 'a'];
            for (var c = 0; c < 26; ++c)
            {
                ++counts[c];
                var data = new Data(counts);
                if (dic.ContainsKey(data)) dic[data].Add(i);
                else dic[data] = new List<int>{ i };
            }
        }
        foreach (var kv in dic)
        {
            if (kv.Value.Count == 1)
            {
                var ans = new List<char>();
                for (var c = 0; c < kv.Key.List.Length; ++c) for (var i = 0; i < kv.Key.List[c]; ++i) ans.Add((char)(c + 'a'));
                WriteLine(string.Concat(ans));
                return;
            }
        }
        WriteLine(-1);
    }
    class Data
    {
        public int Key;
        public int[] List;
        public Data(int[] list)
        {
            List = (int[]) list.Clone();
            var k = 0L;
            foreach (var li in list) k = (k * 11 + li) % 1_000_000_007;
            Key = (int)k;
        }
        public override bool Equals(object obj)
        {
            var b = obj as Data;
            if (Key != b.Key) return false;
            for (var i = 0; i < List.Length; ++i) if (List[i] != b.List[i]) return false;
            return true;
        }
        public override int GetHashCode()
        {
            return Key;
        }
    }
}