#define _CRT_SECURE_NO_WARNINGS
#include<sstream>
#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<climits>
#include<cmath>
#include<string>
#include<vector>
#include<set>
#include<map>
#include<queue>
#include<numeric>
#include<functional>
#include<algorithm>
#include<bitset>
#include<tuple>
#include<unordered_set>
#include<random>
using namespace std;
#define INF (1<<29)
#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define all(v) v.begin(),v.end()
#define uniq(v) v.erase(unique(all(v)),v.end())
#define indexOf(v,x) (find(all(v),x)-v.begin())


string cmp(string s,string t){
	if(s.size()<t.size())return t;
	if(s.size()>t.size())return s;
	rep(i,s.size()){
		if(s[i]==t[i])continue;
		if(s[i]=='4'&&t[i]=='7')return s;
		if(s[i]=='7'&&t[i]=='4')return t;
		if(s[i]<t[i])return t;
		return s;
	}
	return s;
}


int main() {
	ios::sync_with_stdio(0);
	cin.tie(0);
	string s,t;
	cin>>s>>t;
	cout<<cmp(s,t)<<endl;
	
	return 0;
}