#include <iostream>
using namespace std;
int main(){
    string a,b;cin>>a>>b;

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