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


int main(){
	ios::sync_with_stdio(false);
	cin.tie(0);
	
	string a, b; cin >> a >> b;
	if(a.size() > b.size()) {
		cout << a << endl;
		return 0;
	}
	if(b.size() > a.size()) {
		cout << b << endl;
		return 0;
	}
	for(int i = 0; i < a.size(); i++) {
		if(a[i] != b[i]){
			if(!((a[i] == '4' && b[i] == '7') ||(a[i] == '7' && b[i] == '4')))  break;
			
		}
		if(a[i] == '4' && b[i] == '7') {
			cout << a << endl;
			return 0;
		}
		if(a[i] == '7' && b[i] == '4') {
			cout << b << endl;
			return 0;
		}
	}
	long long s = stoi(a), t = stoi(b);
	cout << max(s, t) << endl;
	
	
	return 0;	
}