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

namespace yukicoder
{
	public class Program
	{
		public static List<int> N = new List<int>();
		public static void Main()
		{
			var a = Console.ReadLine().Trim().Split(' ').Select(value => int.Parse(value)).Distinct().ToArray();
			var c = false;
            if (a[0].ToString().Length == a[1].ToString().Length)
			{
				a = fs(a);
				c = true;
			}
			var k = Math.Max(a[0], a[1]);
            if (c)
            {
				var K = k.ToString();
				foreach(var n in N)
                {
					if (K[n] == '4')
					{
						K = K.Remove(n, 1).Insert(n, "7");
					}
					else if(K[n] == '7')
                    {
						K = K.Remove(n, 1).Insert(n, "4");
					}
                }
				k = int.Parse(K);
            }
			Console.WriteLine(k);
		}
		static int[] fs(int[] a)
		{
			var k = a.Select(value =>value.ToString()).ToArray();
			var A = new string[2];
			for(var i = 0; i < k[0].Length; i++)
            {
                if (k[0][i] == '4' && k[1][i] == '7')
                {
					A[0] += "7";
					A[1] += "4";
					N.Add(i);
                }
				else if(k[0][i] == '7' && k[1][i] == '4'){
					A[0] += "4";
					A[1] += "7";
					N.Add(i);
                }
                else
                {
					A[0] += k[0][i];
					A[1] += k[1][i];
                }
            }
			a = A.Select(value => int.Parse(value)).ToArray();
			return a;
		}
	}
}