import java.util.*;

public class Yuki293 {
    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 a, String b) {
        char[] l_a = a.toCharArray();
        char[] l_b = b.toCharArray();
        if (l_a.length != l_b.length) {
            return l_a.length > l_b.length;
        }
        int n = l_a.length;
        for (int i = 0; i < n; i++) {
            if (l_a[i] != l_b[i]) {
                if (l_a[i] == '4' && l_b[i] == '7' || l_a[i] == '7' && l_b[i] == '4') {
                    return l_a[i] < l_b[i];
                } else {
                    return l_a[i] > l_b[i];
                }
            }
        }
        return true;
    }
}