import java.io.*;
import java.util.*;

class Main {
	
	public static void out (Object o) {
		System.out.println(o);
	}
	
	public static String max (String a , String b) {
		if (a.length() != b.length())
			return a.length() > b.length() ? a : b;
		for (int i = 0; i < a.length(); i++) {
			int n = a.charAt(i) - '0';
			int m = b.charAt(i) - '0';
			if (n == 4 && m == 7) {
				return a;
			} else if (n == 7 && m == 4) {
				return b;
			} else if (n != m) {
				return n > m ? a : b;
			}
		}
		return a;
	}
	
	public static void main (String[] args) throws IOException {
		BufferedReader br = 
			new BufferedReader(new InputStreamReader(System.in));
		
		String[] line = br.readLine().split(" ");
		
		out(max(line[0] , line[1]));
	}
}