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

namespace AOJ
{
    class Tuple
    {
        public int Item1 { get; private set; }
        public int Item2 { get; private set; }

        public Tuple(int item1, int item2)
        {
            Item1 = item1;
            Item2 = item2;
        }
    }

    class Program
    {
        static List<Tuple> getInput()
        {
            var input = new List<Tuple>();
            String str;
            String[] strs;

            while ((str = Console.ReadLine()) != null)
            {
                strs = str.Split(' ');
                input.Add(new Tuple(int.Parse(strs[0]), int.Parse(strs[1])));
            }

            return input;
        }

        static String solve(List<Tuple> inputs)
        {
            bool flag = true;
            foreach (var input in inputs)
            {
                flag &= inputs
                    .Where(i => (i.Item1 == input.Item1 && i.Item2 == input.Item2) || (i.Item1 == input.Item2 && i.Item2 == input.Item1))
                    .Count() % 2 == 0;
            }

            return (flag)? "yes" : "no";
        }

        static void Main(string[] args)
        {
            var inputs = getInput();
            Console.WriteLine(solve(inputs));
        }
    }
}