import java.util.Scanner;


public class Main_yukicoder293 {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        char[] a = sc.next().toCharArray();
        char[] b = sc.next().toCharArray();

        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++) {
        		if (a[i] != b[i]) {
        			if (a[i] == '4' && b[i] == '7') {
        	        	System.out.println(a);
        	        	break;
        			} else if (a[i] == '7' && b[i] == '4') {
        	        	System.out.println(b);
        	        	break;
        			} else if (a[i] > b[i]) {
        	        	System.out.println(a);
        	        	break;
        			} else {
        	        	System.out.println(b);
        	        	break;
        			}
        		}
        	}
        }

        sc.close();
    }
}