#include <bits/stdc++.h>
using namespace std;

#define lp(i, a, b) for (int i = (int)(a); i < (int)(b); i++)
#define rep(i, n) lp(i, 0, n)
#define rlp(i, a, b) for (int i = (int)(b) - 1; i >= (int)(a); i--)
#define rrep(i, n) rlp(i, 0, n)

#define mp make_pair
#define mt make_tuple
#define pb push_back
#define eb emplace_back
#define all(c) begin(c), end(c)

typedef long long ll;
typedef unsigned long long ull;

template<typename T>
inline void unique(T &c){
	sort(all(c));
	c.erase(unique(all(c)), end(c));
}

int main()
{
	string s, t;
	cin >> s >> t;
	if (s.size() != t.size()){
		if (s.size() < t.size()) cout << t << endl;
		else cout << s << endl;
	}
	else {
		string r;
		rep(i, s.size()){
			if (s[i] == '4' && t[i] == '7') r = s;
			else if (s[i] == '7' && t[i] == '4') r = t;
			else if (s[i] > t[i]) r = s;
			else if (s[i] < t[i]) r = t;
			if (r.size()) break;
		}
		if (!r.size()) r = s;
		cout << r << endl;
	}
}