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

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner();
        char[] a = sc.next().toCharArray();
        char[] b = sc.next().toCharArray();
        if (check(a, b) < 0) {
            System.out.println(b);
        } else {
            System.out.println(a);
        }
    }
    
    static int check(char[] a, char[] b) {
        if (a.length == b.length) {
            for (int i = 0; i < a.length; i++) {
                if (a[i] != b[i]) {
                    if (a[i] == '4' && b[i] == '7') {
                        return 1;
                    } else if (a[i] == '7' && b[i] == '4') {
                        return -1;
                    } else {
                        return a[i] - b[i];
                    }
                }
            }
            return 0;
        } else {
            return a.length - b.length;
        }
    }
} 
class Utilities {
    static String arrayToLineString(Object[] arr) {
        return Arrays.stream(arr).map(x -> x.toString()).collect(Collectors.joining("\n"));
    }
    
    static String arrayToLineString(int[] arr) {
        return String.join("\n", Arrays.stream(arr).mapToObj(String::valueOf).toArray(String[]::new));
    }
}
class Scanner {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    StringTokenizer st = new StringTokenizer("");
    StringBuilder sb = new StringBuilder();
    
    public Scanner() throws Exception {
        
    }
    
    public int nextInt() throws Exception {
        return Integer.parseInt(next());
    }
    
    public long nextLong() throws Exception {
        return Long.parseLong(next());
    }
    
    public double nextDouble() throws Exception {
        return Double.parseDouble(next());
    }
    
    public int[] nextIntArray() throws Exception {
        return Stream.of(br.readLine().split(" ")).mapToInt(Integer::parseInt).toArray();
    }
    
    public String next() throws Exception {
        while (!st.hasMoreTokens()) {
            st = new StringTokenizer(br.readLine());
        }
        return st.nextToken();
    }
    
}