#define _USE_MATH_DEFINES
 
#include <iostream>
#include <iomanip>
#include <sstream>
 
#include <algorithm>
#include <cmath>
 
#include <string>
#include <queue>
#include <vector>
#include <complex>
#include <set>
#include <map>
#include <stack>
#include <list>
 
/////////
#define REP(i, x, n) for(int i = x; i < n; i++)
#define rep(i,n) REP(i,0,n)
#define P(p) cout<<(p)<<endl;
 
#define PII pair<int,int>
/////////
typedef long long LL;
typedef long double LD;
/////////
using namespace::std;
/////////
/////////

void solve(){
	string A,B;
	cin >> A >> B;
	int lenA,lenB;
	lenA = A.size();
	lenB = B.size();
	if( lenA > lenB ){
		cout << A << endl;return;
	}else if( lenB > lenA ){
		cout << B << endl;return;
	}
	for(int i=0;i<lenA;++i){
		if( A[i] == '4' && B[i] == '7' ){
			cout << A << endl;
			break;
		}else if( A[i] == '7' && B[i]== '4' ){
			cout << B << endl;
			break;
		}else if( A[i] > B[i] ){
			cout << A << endl;
			break;
		}else if( B[i] > A[i] ){
			cout << B << endl;
			break;
		}
	}
	//A==B
}

int main(void){
    std::cin.tie(0); 
    std::ios::sync_with_stdio(false);
    std::cout << std::fixed;//
    //cout << setprecision(16);//
	
	//cpp
	solve();
	return 0;
}