#include<iostream>
#include<string>
using namespace std;

int main(){
    string a,b;
    cin >> a >> b;
    if(a.size()==b.size()){
        for(int i=0;i<a.size();i++){
            if(a[i]==b[i]) continue;
            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;
            }
        }
    }
    int x=atoi(a.c_str());
    int y=atoi(b.c_str());
    if(x>y) cout << a << endl;
    else cout << b << endl;
}