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

class Program {
    static void Main(string[] args) {
        string s = Console.ReadLine();
        int A = int.Parse(s.Split(' ')[0]);
        int B = int.Parse(s.Split(' ')[1]);

        //Aの桁数
        int keta_A = (int)Math.Floor(Math.Log10(A)) + 1;
        //Bの桁数
        int keta_B = (int)Math.Floor(Math.Log10(B)) + 1;

        //Console.WriteLine(A.ToString()[0]);
        //Console.WriteLine(B.ToString()[0]);

        //桁が異なるとき
        if (keta_A > keta_B) {
            Console.WriteLine(A);
        } else if (keta_A < keta_B) {
            Console.WriteLine(B);
        //以下、桁が同じとき
        } else {
            for (int i = 0; i < keta_A; i++) {
                if ((A.ToString()[i] == '4' && B.ToString()[i] == '7')) {
                    //Console.WriteLine("1");
                    Console.WriteLine(A);
                    break;
                } else if (A.ToString()[i] == '7' && B.ToString()[i] == '4') {
                    //Console.WriteLine("2");
                    Console.WriteLine(B);
                    break;
                } else if (A.ToString()[i] == B.ToString()[i]) {
                    //Console.WriteLine("3");
                    //次のループに入る
                } else {
                    //Console.WriteLine("4");
                    Console.WriteLine(Math.Max(A, B));
                    break;
                }
            }
        }
    }
}