using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;

namespace D2._2
{
    class Program
    {
        static void Main(string[] args)
        {
            if (args.Length < 1) return;
            if (File.Exists(args[0]) == false) return;

            var file = File.OpenText(args[0]);

            var list = new List<string>();
            string code = null;
            do
            {
                var line = file.ReadLine();
                if (line == null) break;

                foreach (var l in list)
                {
                    int diffCars = 0;
                    string c = string.Empty;
                    for (int i = 0; i < l.Length; i++)
                    {
                        if (l[i] != line[i]) diffCars++;
                        else c += l[i];

                        if (diffCars >= 2) break;
                    }
                    if (diffCars == 1)
                    {
                        code = c;
                        break;
                    };
                }

                list.Add(line);
            } while (code == null);

            Console.WriteLine(code);
        }
    }
}