#include <iostream>
#include <string>
using namespace std;
int main(int argc, char* argv[])
{
	string S1,S2;
	cin>>S1>>S2;
	int sLen1=S1.size();
	int sLen2=S2.length();
	if (sLen1>sLen2){
		cout<<S1<<endl;
		return 0;
	}else if (sLen1<sLen2){
		cout<<S2<<endl;
		return 0;
	}
	for (int i=0;i<sLen1;i++){
		if (S1[i]!=S2[i]){
			if (S1[i]=='4' && S2[i]=='7'){
				cout<<S1<<endl;
				return 0;
			}
			if (S1[i]=='7' && S2[i]=='4'){
				cout<<S2<<endl;
				return 0;
			}
			if (S1[i]>S2[i]){
				cout<<S1<<endl;
				return 0;
			}
			if (S2[i]>S1[i]){
				cout<<S2<<endl;
				return 0;
			}
		}
	}
	cout<<S1<<endl;
	return 0;
}