import java.util.*;

public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    String a = sc.next();
    String b = sc.next();
    String A = "";
    String B = "";
    for(int i = 0; i < a.length(); i++) {
      char c = a.charAt(i);
      if(c == '4') A += "7";
      if(c == '7') A += "4";
      if(c != '4' && c != '7') A += String.valueOf(c);
    }
    for(int i = 0; i < b.length(); i++) {
      char c = b.charAt(i);
      if(c == '4') B += "7";
      if(c == '7') B += "4";
      if(c != '4' && c != '7') B += String.valueOf(c);
    }
    String ans = a;
    if(Long.parseLong(A) < Long.parseLong(B)) ans = b;
    System.out.println(ans);
  }
}