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

namespace D7._1
{
    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 builder = new Dictionary<char, Node>();

            do
            {
                var line = file.ReadLine();
                if (line == null) break;

                AddDependency(builder, line);

            } while (true);

            string path = GetPath(builder);

            Console.WriteLine(path);
        }

        private static string GetPath(Dictionary<char, Node> builder)
        {
            List<Node> possibleValues = builder.Select(s => s.Value).ToList();

            string path = "";
            while (possibleValues.Count() > 0)
            {
                var first = possibleValues
                    .OrderBy(c => c.Name)
                    .FirstOrDefault(c => c.Dependencies.Count == 0);

                path += first.Name;

                RemoveDependency(possibleValues, first);
            }

            return path;
        }

        private static void RemoveDependency(List<Node> possibleValues, Node first)
        {
            possibleValues.Remove(first);
            foreach (var next in possibleValues)
                next.Dependencies.Remove(first);
        }

        private static void AddDependency(Dictionary<char, Node> builder, string line)
        {
            var left = line.Substring(5, 1)[0];
            var right = line.Substring(36, 1)[0];

            if (builder.ContainsKey(left) == false)
                builder.Add(left, new Node
                {
                    Name = left,
                    Dependencies = new List<Node>()
                });

            if (builder.ContainsKey(right) == false)
                builder.Add(right, new Node
                {
                    Name = right,
                    Dependencies = new List<Node>()
                });

            var nleft = builder[left];
            var nright = builder[right];

            nright.Dependencies.Add(nleft);
        }

        public class Node
        {
            public char Name { get; set; }

            public List<Node> Dependencies { get; set; }

            public override int GetHashCode()
            {
                return Name.GetHashCode();
            }

            public override bool Equals(object obj)
            {
                var o = obj as Node;
                return Name.Equals(o.Name);
            }
        }
    }
}