using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Yuki
{
    class Program
    {
        static void Main(string[] args)
        {
            string[] a = Console.ReadLine().Split();
            int b = a[0].Length > a[1].Length ? a[0].Length - 1 : a[1].Length - 1;
            string w0 = a[0];
            string w1 = a[1];


            for (int i = b; i > -1; i--)
            {
                if ((a[0][i] == '4') && (a[1][i] == '7'))
                {
                    a[0] = a[0].Insert(i, "7");
                    a[0] = a[0].Remove(i + 1, 1);
                    a[1] = a[1].Insert(i, "4");
                    a[1] = a[1].Remove(i + 1, 1);

                    continue;
                }
                if ((a[0][i] == '7') && (a[1][i] == '4'))
                {
                    a[0] = a[0].Insert(i, "4");
                    a[0] = a[0].Remove(i + 1, 1);
                    a[1] = a[1].Insert(i, "7");
                    a[1] = a[1].Remove(i + 1, 1);

                }
            }

            string ans = int.Parse(a[0]) > int.Parse(a[1]) ? w0 : w1;
            Console.Write(ans);
            Console.ReadLine();
        }

        static int[] ConvertStringArrayToIntArray(string[] array)
        {
            return Array.ConvertAll(array, str => int.Parse(str));
        }

        static List<int> ConvertStringArrayToIntList(string[] str)
        {
            var list = new List<int>();
            foreach (var c in str) list.Add(int.Parse(c));
            return list;
        }
    }
}