#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <map>

using namespace std;

#define INF 		2000000007
#define MOD			1000003

#define MAX 		105

#define REP(i,n)	for(int i=0;i<(n);++i)
#define REPS(i,s,t)	for(int i=(s);i<(t);++i)

typedef unsigned int 			uint;
typedef unsigned long long int	ull;


int main(){
	string a; cin >> a;
	string b; cin >> b;

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

	return 0;
}