using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.Text;
using System.Text.RegularExpressions;
using System.Linq;

class Magatro
{
    static void Main()
    {
        string[] AB = Console.ReadLine().Split(' ');
        if (Compare(AB[0], AB[1])==1)
        {
            Console.WriteLine(AB[0]);
        }
        else
        {
            Console.WriteLine(AB[1]);
        }
    }
    static int Compare(string A,string B)
    {
        if (A.Length > B.Length)
        {
            return 1;
        }
        else if (A.Length < B.Length)
        {
            return -1;
        }
        for(int i = 0; i < A.Length; i++)
        {
            if (Com(A[i], B[i]) == 1)
            {
                return 1;
            }
            else if (Com(A[i], B[i]) == -1)
            {
                return -1;
            }
            
        }
        return 0;
    }
    static int Com(char a,char b)
    {
        if (a == '4' && b == '7')
        {
            return 1;
        }
        else if (a == '7' && b == '4')
        {
            return -1;
                }
        else
        {
            if (a > b)
            {
                return 1;
                }
            else if(a<b)
            {
                return -1;
            }
            else
            {
                return 0;
            }
        }
    }
  
}