package no293;

import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		String a = sc.next();
		String b = sc.next();
		System.out.println(less(a,b) ? a : b);
	}

	public static boolean less(String aa,String bb) {
		char[] a = aa.toCharArray();
		char[] b = bb.toCharArray();
		if (a.length != b.length) {
			return a.length > b.length;
		}
		int n = a.length;
		for(int i=0;i<n;i++) {
			if (a[i] != b[i]) {
				if (a[i] == '4' && b[i] == '7' || a[i] == '7' && b[i] == '4') {
					return a[i] < b[i];
				}else{
					return a[i] > b[i];
				}
			}
		}
		return true;
	}

}