import java.util.Arrays;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.Scanner;
import java.util.Set;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		final long A = sc.nextLong();
		final long B = sc.nextLong();
		
		final char[] A_ch = Long.toString(A).toCharArray();
		final char[] B_ch = Long.toString(B).toCharArray();
		
		if(A_ch.length < B_ch.length){
			System.out.println(B);
		}else if(A_ch.length > B_ch.length){
			System.out.println(A);
		}else{
			for(int i = 0; i < A_ch.length; i++){
				if(A_ch[i] == '4' && B_ch[i] == '7'){
					System.out.println(A);
					break;
				}else if(A_ch[i] == '7' && B_ch[i] == '4'){
					System.out.println(B);
					break;
				}else if(A_ch[i] > B_ch[i]){
					System.out.println(A);
					break;
				}else if(A_ch[i] < B_ch[i]){
					System.out.println(B);
					break;
				}
			}
		}
		
	}

}