#include <bits/stdc++.h>


using namespace std;
typedef long long ll;

#define REP(i,n) for(ll i=0; i<ll(n); i++)
#define FOR(i,m,n) for(ll i=ll(m); i<ll(n); i++)
#define ALL(obj) (obj).begin(),(obj).end()
#define VI vector<int>
#define VP vector<pair<int,int>>
#define VPP vector<pair<int,pair<int,int>>>
#define VLL vector<long long>
#define VVI vector<vector<int>>
#define VVLL vector<vector<long long>>
#define VC vector<char>
#define VS vector<string>
#define VVC vector<vector<char>>
#define VB vector<bool>
#define VVB vector<vector<bool>>
#define fore(i,a) for(auto &i:a)
typedef pair <int, int> P;
template<typename T> using min_priority_queue = priority_queue<T, vector<T>, greater<T>>;

const int  INF = 1 << 29;
const ll INFL = 1LL << 60;
const ll mod = 1000000007;




int main() {
	
	string s, t;
	cin >> s >> t;

	if (s.size() != t.size()) {
		if (s.size() > t.size())cout << s << endl;
		else cout << t << endl;
		return 0;
	}

	REP(i, s.size()) {
		if (s[i] == t[i])continue;
		if (s[i] > t[i]) {
			if (s[i] == '7'&&t[i] == '4')cout << t << endl;
			else cout << s << endl;
		}
		else {
			if (s[i] == '4'&&t[i] == '7')cout << s << endl;
			else cout << t << endl;
		}
		return 0;
	}



}