#include<bits/stdc++.h>
using lint=long long;
int main(){
    std::cin.tie(nullptr);std::ios_base::sync_with_stdio(false);
    std::cout.setf(std::ios_base::fixed);std::cout.precision(15);
    std::string s,t;std::cin>>s>>t;
    std::cout<<((s.length()!=t.length()
        ?s.length()<t.length()
        :std::lexicographical_compare(s.begin(),s.end(),t.begin(),t.end(),[](char c,char d){
            if(c=='4'&&d=='7')return false;
            if(c=='7'&&d=='4')return true;
            return c<d;
        }))?t:s)<<'\n';
}