#include<bits/stdc++.h>
using namespace std;

int main(){
    string a,b;
    cin>>a>>b;
    if(a.size()>b.size())cout<<a<<endl;
    else if(a.size()<b.size())cout<<b<<endl;

    else{
        for(int i=0;i<a.size();i++){
            if(a[i]==b[i])continue;
            if(a[i]=='4'&&b[i]=='7'){
                cout<<a<<endl;
                break;
            }
            if(a[i]=='7'&&b[i]=='4'){
                cout<<b<<endl;
                break;
            }
            if(a[i]>b[i]){
                cout<<a<<endl;
                break;
            }
            if(b[i]>a[i]){
                cout<<b<<endl;
                break;
            }
        }
    }

    return 0;
}