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

public class Main {
    public static void main(String[] args) throws Exception {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String[] first = br.readLine().split(" ", 2);
        char[] aArr = first[0].toCharArray();
        char[] bArr = first[1].toCharArray();
        if (aArr.length != bArr.length) {
            if (aArr.length > bArr.length) {
                System.out.println(aArr);
            } else {
                System.out.println(bArr);
            }
            return;
        }
        for (int i = 0; i < aArr.length; i++) {
            if (aArr[i] == bArr[i]) {
                continue;
            }
            if (aArr[i] == '4' && bArr[i] == '7') {
                System.out.println(aArr);
            } else if (aArr[i] == '7' && bArr[i] == '4') {
                System.out.println(bArr);
           } else if (aArr[i] > bArr[i]) {
                System.out.println(aArr);
            } else {
                System.out.println(bArr);
            }
            return;
        }
    }
}