import java.util.*;
import java.math.*;
import java.awt.geom.*;
import java.io.*;
    
    
public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		String a = sc.next();
		String b = sc.next();
		int ans = compare(a,b);
		
		if(ans > 0) {
			System.out.println(a);
		}
		else if (ans < 0){
			System.out.println(b);
		}
	}
	
	static int compare(String a, String b) {
		if(a.equals(b)) return 0;
		if(a.length() > b.length()) return 1;
		else if(a.length() < b.length()) return -1;
		else {
			for(int i = 0; i < a.length(); i++) {
				if(a.charAt(i) == b.charAt(i)) continue;
				if(a.charAt(i) == '4' && b.charAt(i) == '7') {
					return 1;
				}
				if(a.charAt(i) == '7' && b.charAt(i) == '4') {
					return -1;
				}
				return Integer.parseInt(String.valueOf(String.valueOf(a.charAt(i)))) - Integer.parseInt(String.valueOf(String.valueOf(b.charAt(i))));
			}
		}
		return 0;
		
	}
	
	
}