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();
		if (a.length() > b.length()) {
		    System.out.println(a);
		} else if (a.length() < b.length())  {
		    System.out.println(b);
		} else {
		    for (int i = 0; i < a.length(); i++) {
		        char ca = a.charAt(i);
		        char cb = b.charAt(i);
		        if ((ca == '4' && cb == '7') || ca > cb) {
		            System.out.println(a);
		            return;
		        } else if ((ca == '7' && cb == '4') || ca < cb) {
		            System.out.println(b);
		            return;
		        }
		    }
		}
	}
}