#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;
			else if(a[i] == '4' and b[i] == '7'){
				cout << a << endl;
				return 0;
			}
			else if(a[i] == '7' and b[i] == '4'){
				cout << b << endl;
				return 0;
			}
			else{
				cout << (a[i] > b[i] ? a : b) << endl;
				return 0;
			}
			
		}
	}
	return 0;
}