#include <bits/stdc++.h>
using namespace std;
string s, t;
int main() {
	cin >> s >> t; string a = s, b = t;
	for(int i = 0; i < s.size(); i++) {
		if(s[i] == '4') s[i] = '7';
		else if(s[i] == '7') s[i] = '4';
	}
	for(int i = 0; i < t.size(); i++) {
		if(t[i] == '4') t[i] = '7';
		else if(t[i] == '7') t[i] = '4';
	}
	int r1 = stoi(s), r2 = stoi(t);
	if(r1 > r2) cout << a << endl;
	else cout << b << endl;
	return 0;
}