#include <bits/stdc++.h>
#include <iostream>
#include <iomanip>
#include <vector>
#include <algorithm>
#include <string>
#include <sstream>
#include <cmath>
#include <stack>
#include <queue>
#include <cctype>
#include <stdio.h>
#include <map>
#include <string.h>
#include <utility>

typedef long long ll;
using namespace std;

typedef pair<int, int> P;


int main() {

	string s,ss;
	cin>>s>>ss;

	bool s_judge =false,ss_judge=false;

	if(s.find('4') != string::npos || s.find('7') != string::npos){
		s_judge =true;
	}

	if(ss.find('4') != string::npos || ss.find('7') != string::npos){
		ss_judge =true;
	}

	if(s.length()<ss.length()){
		cout<<ss<<endl;
	}else if (s.length()>ss.length()) {
		cout<<s<<endl;
	}else {
	if(s_judge && ss_judge){

		for(int i=0; i<s.length();i++){
			if(s[i]-ss[i] !=0){
				if(s[i] =='4'&& ss[i]=='7'){
					cout<<s<<endl;
					return 0;
				}else if(s[i] =='7'&& ss[i]=='4'){
					cout<<ss<<endl;
					return 0;
				}else {
					if(s<ss){
							cout<<ss<<endl;
							return 0;
						}else if (s>ss) {
							cout<<s<<endl;
							return 0;
						}
				}
			}
		}
	}else {
			if(s<ss){
				cout<<ss<<endl;
			}else if (s>ss) {
				cout<<s<<endl;
			}
	}
	}

	return 0;
}