package main

import (
	"bufio"
	"os"
	"strconv"
	"fmt"
)

func main() {
	A := next()
	B := next()

	if len(A) > len(B) {
		fmt.Println(A)
		return
	}else if len(A) < len(B) {
		fmt.Println(B)
		return
	}
	for i := 0; i < len(A); i++ {
		if A[i] == B[i] {
			continue
		}
		if A[i] == '7' && B[i] == '4' {
			fmt.Println(B)
			return
		}else if A[i] == '4' && B[i] == '7' {
			fmt.Println(A)
			return
		}else if A[i] < B[i] {
			fmt.Println(B)
			return
		}else {
			fmt.Println(A)
			return
		}
	}

}

var s = bufio.NewScanner(os.Stdin)

func next() string {
	s.Split(bufio.ScanWords)
	s.Scan()
	return s.Text()
}
func nextLine() string {
	s.Split(bufio.ScanLines)
	s.Scan()
	return s.Text()
}

func nextInt() int {
	i, e := strconv.Atoi(next())
	if e != nil {
		panic(e)
	}
	return i
}
func nextLong() int64 {
	i, e := strconv.ParseInt(next(), 10, 64)
	if e != nil {
		panic(e)
	}
	return i
}


func min(a int, b int) int {
	if a < b {
		return a
	}
	return b
}
func max(a int, b int) int {
	if a < b {
		return b
	}
	return a
}