import java.util.*;

public class Exercise80{
  public static void main (String[] args){

    Scanner sc = new Scanner(System.in);

    String one = sc.next();
    String two = sc.next();

    if(one.length() == two.length()){
      String[] arrayOne = one.split("");
      String[] arrayTwo = two.split("");

      compare(one, arrayOne, two, arrayTwo);
    }else{
      if (one.length() > two.length()){
        System.out.println(one);
      }else{
        System.out.println(two);
      }
    }
  }

  private static void compare(String one, String[] arrayOne, String two, String[] arrayTwo){
    for(int i = 0; i < arrayOne.length; i++){
      if(arrayOne[i].equals("4") && arrayTwo[i].equals("7")){
        arrayOne[i] = "7";
        arrayTwo[i] = "4";
      }else if(arrayOne[i].equals("7") && arrayTwo[i].equals("4")){
        arrayOne[i] = "4";
        arrayTwo[i] = "7";
      }
    }

    String newOne = String.join("", arrayOne);
    String newTwo = String.join("", arrayTwo);

    if(Integer.parseInt(newOne) > Integer.parseInt(newTwo)){
      System.out.println(one);
    }else{
      System.out.println(two);
    }
  }
}