import java.util.Scanner;
import java.util.regex.Pattern;
import java.util.regex.Matcher;

public class FourIsLargerThanSeven {
    static Scanner scanner = new Scanner(System.in);
    public static void main(String[] args){
        char[] A = Input(1 , 2000000000);
        char[] B = Input(1 , 2000000000);
        int Answer ;
        if(A.length > B.length){
            String Character = String.valueOf(A);
            Answer = Integer.valueOf(Character);
        }else if(A.length < B.length){
            String Character = String.valueOf(B);
            Answer = Integer.valueOf(Character);
        }else{
            Answer = Comparison(A,B);
        }
        System.out.println(Answer);
    }
    
    public static char[] Input(int Min , int Max){
        String TheNumber = scanner.next();
        try{
            Pattern pattern = Pattern.compile("[^0-9]");
            Matcher matcher = pattern.matcher(TheNumber);
            if(matcher.find()){
                System.out.println("数字を入力してください");
                System.exit(0);
            }
            if(Integer.valueOf(TheNumber) < Min || Integer.valueOf(TheNumber) > Max){
                System.out.println(Min + "以上" + Max + "以下で入力してください");
                System.exit(0);
            }
        }catch(Exception E){
            System.out.println("想定外のエラーです");
            System.exit(0);
        }
        char[] Number = TheNumber.toCharArray();
        return Number;
    }
    
    public static int Comparison(char[] A , char[] B){
        int Answer = 0;
        for(int i = 0 ; i < A.length ; i++){
            if(A[i] > B[i]){
                if(A[i] == '7' && B[i] == '4'){
                    Answer = Integer.valueOf(String.valueOf(B));
                    return Answer;
                }
            }else if(A[i] < B[i]){
                if(A[i] == '4' && B[i] == '7'){
                    Answer = Integer.valueOf(String.valueOf(A));
                    return Answer;
                }
            }
        }
        int NumberA = Integer.valueOf(String.valueOf(A));
        int NumberB = Integer.valueOf(String.valueOf(B));
        if(NumberA > NumberB){
            Answer = NumberA;
        }else{
            Answer = NumberB;
        }
        
        return Answer;
    }
}