#define _CRT_SECURE_NO_WARNINGS
#include <iostream>
#include <climits>
#include <cmath>
#include <string>
#include <vector>
#include <algorithm>
#include <queue>
#include <map>
#include <functional>
#include <set>
#include <numeric>
#include <stack>
#include <utility>
#include <time.h>
#include <iterator>
//#include "util.h"

using namespace std;
typedef unsigned uint;
typedef long long ll;
typedef unsigned long long ull;



//呪文
template <typename _KTy, typename _Ty> ostream& operator << (ostream& ostr, const pair<_KTy, _Ty>& m) { cout << "{" << m.first << ", " << m.second << "}"; return ostr; }
template <typename _KTy, typename _Ty> ostream& operator << (ostream& ostr, const map<_KTy, _Ty>& m) { if (m.empty()) { cout << "{ }"; return ostr;	} cout << "{" << *m.begin(); for (auto itr = ++m.begin(); itr != m.end(); itr++) { cout << ", " << *itr; } cout << "}"; return ostr; }
template <typename _Ty> ostream& operator << (ostream& ostr, const vector<_Ty>& v) { if (v.empty()) { cout << "{ }"; return ostr; }	cout << "{" << v.front(); for (auto itr = ++v.begin(); itr != v.end(); itr++) { cout << ", " << *itr; }	cout << "}"; return ostr; }
template <typename _Ty> ostream& operator << (ostream& ostr, const set<_Ty>& s) { if (s.empty()) { cout << "{ }"; return ostr; } cout << "{" << *(s.begin()); for (auto itr = ++s.begin(); itr != s.end(); itr++) { cout << ", " << *itr; }	cout << "}"; return ostr; }
#define PI 3.14159265358979323846
#define EPS 1e-6
#define MIN(a,b) ((a)<(b)?(a):(b))
#define MAX(a,b) ((a)>(b)?(a):(b))
#define all(x) (x).begin(), (x).end()



int yuki0293()
{
	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] == B[i]) continue;

		if (A[i] == '4' && B[i] == '7') {
			cout << A << endl;
			return 0;
		}
		if (A[i] == '7' && B[i] == '4') {
			cout << B << endl;
			return 0;
		}
		cout << (A[i] > B[i] ? A : B) << endl;
		return 0;
	}

	return 0;
}

int main()
{
	//clock_t start, end;
	//start = clock();

	yuki0293();
	
	//end = clock();
	//printf("%d msec.\n", end - start);

	return 0;
}