package main

import (
	"fmt"
)

func main() {
	var A, B string
	fmt.Scan(&A, &B)
	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
		}
	}

}