#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 conv(string s){
	rep(i,s.size()){
		if(s[i]=='4'){
			s[i]='7';
		}else if(s[i]=='7')s[i]=' ';
	}
	rep(i,s.size())
		if(s[i]==' ')s[i]='4';
	return s;
}


int main() {
	ios::sync_with_stdio(0);
	cin.tie(0);
	string s,t;
	cin>>s>>t;
	s=conv(s);
	t=conv(t);
	if(s.size()<t.size()||s<t)cout<<conv(t)<<endl;
	else cout<<conv(s)<<endl;
	
	return 0;
}