//4を7に、7を4に置き換えて良さそう→ダメ

#include <string>
#include <iostream>
using namespace std;

//aがbより小さければ-1, 大きければ1を返す
int compare( string a, string b ){
	if( a.length() > b.length() )
		return 1;
	if( a.length() < b.length() )
		return -1;
	
	int n = a.length();
	for( int i = 0; i < n; i++ ){
		if( a[i] != b[i] ){
			if( a[i] == '4' && b[i] == '7' )
				return 1;
			if( a[i] == '7' && b[i] == '4' )
				return -1;
			if( a[i] > b[i] )
				return 1;
			if( a[i] < b[i] )
				return -1;
		}
	}
	return 0;	//同じ
}

int main()
{
	string a, b;
	cin >> a >> b;
	if( compare(a, b) == 1 )
		cout << a << endl;
	else
		cout << b << endl;
	return 0;
}