#include <iostream>
#include <vector>
#include <cstdio>
#include <sstream>
#include <map>
#include <string>
#include <algorithm>
#include <queue>
#include <cmath>
#include <set>
using namespace std;

int main(){
	string a,b;
	cin >> a >> b;
	if(a.size() != b.size()){
		cout << (a.size()>b.size()?a:b) << endl;
		return 0;
	}

	for(int i=0; i<a.size(); 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[i] > b[i]?a:b) << endl;
			return 0;
		}
	}
	return 0;
}