#include <iostream>
#include <algorithm>
#include <functional>
#include <string>
#include <climits>
#include <vector>
#include <numeric>
#include <complex>
#include <map>
#include <bitset>
#include <stack>
#include <queue>
#include <set>
using namespace std;

//#define __int64 long long
#define long __int64
#define REP(i,a,b) for(int i=a;i<b;i++)
#define rep(i,n) REP(i,0,n)
const int Vec = 4;
const int Vecy[Vec] = {0,-1,0,1};
const int Vecx[Vec] = {1,0,-1,0};
const int Modd = 1000000007;

int convert(string in){
	int res = 0;
	for(int i=0; i<in.length(); i++){
		res *= 10;
		if(in[i] == '4'){
			res += 7;
		}else if(in[i] == '7'){
			res += 4;
		}else{
			res += (in[i] - '0');
		}
	}
	
	return res;
}


int main(){
	string ina,inb;
	
	cin >> ina >> inb;
	
	if(convert(ina) >= convert(inb)){
		cout << ina << endl;
	}else{
		cout << inb << endl;
	}
    
    return 0;
}