#include <iostream>
#include <string>
using namespace std;
int main( void )
{
  string a, b;
  cin >> a >> b;
  if( a.size() > b.size() ){
    cout << a << endl;
    return 0;
  } else if( a.size() < b.size() ){
    cout << b << endl;
    return 0;
  }
  for( int i=0; i<a.size(); i++ ){
    if( a[i] == b[i] ) continue;
    if( a[i] > b[i] ){
      if( a[i] == '7' && b[i] == '4' ) cout << b << endl;
      else cout << a << endl;
      return 0;
    }
    if( a[i] == '4' && b[i] == '7' ) cout << a << endl;
    else cout << b << endl;
    return 0;
  }
}