#include <iostream>
#include <cmath>
#include <cstdio>
#include <algorithm>
#include <string>
#include <vector>
#include <queue>
#include <string.h>
#include <map>
#include <fstream>
#include <functional>
#include <iomanip>
#include <bitset>
#include <stack>
#include <set>
#include <climits>
#define MAX_N 2000100
#define LOG 21
#define PI 3.141592653589
#define EPS 1e-10
#define MOD 1000000007
#define YJSNPI 810
#define INF (1 << 30)
#define ADD(a, b) a = (a + (ll)b) % MOD
#define MUL(a, b) a = (a * (ll)b) % MOD
#define MAX(a, b) a = max(a, b)
#define MIN(a, b) a = min(a, b)
#define REP(i, a, b) for(int i = a; i < b; i++)
#define RER(i, a, b) for(int i = a - 1; i >= b; i--)
  
using namespace std;
  
typedef long long ll;
typedef pair<double, int> pi;
typedef pair<int, pi> ppi;
typedef vector<vector<int>> mat;


string solve(const string& s1, const string& s2) {
	if(s1.size() > s2.size()) return s1;
	if(s1.size() < s2.size()) return s2;
	int N = s1.size();
	REP(i, 0, N) {
		int a = s1[i] - '0', b = s2[i] - '0';
		if(a == 4 && b == 7) return s1;
		else if(a == 7 && b == 4) return s2;
		else if(a > b) return s1;
		else if(a < b) return s2;
	}
}

int main() {
	string str1, str2;
	cin >> str1 >> str2;
	cout << solve(str1, str2) << endl;
}