package main

import . "fmt"

func main() {
	var a, b string
	Scan(&a,&b)
	switch {
		case len(a) > len(b):
			Println(a)
		case len(a) < len(b):
			Println(b)
		default:
			for i := 0; i < len(a); i++ {
				if a[i]==b[i] {
					continue
				}
				switch {
					case a[i] == '4' && b[i] == '7':
						Println(a)
					case a[i] == '7' && b[i] == '4':
						Println(b)
					case a[i] > b[i]:
						Println(a)
					case a[i] < b[i]:
						Println(b)
				}
				return
			}
	}
}