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

signed main(){
  string A, B; cin >> A >> B;
  if( A.size() != B.size() )
    cout << ( A.size() > B.size() ? A : B ) << endl,
    exit( 0 );
  for( int i = 0; i < A.size(); ++i )
    if( A[ i ] != B[ i ] ){
      if( A[ i ] == '4' and B[ i ] == '7' )
        cout << A << endl;
      else if( A[ i ] == '7' and B[ i ] == '4' )
        cout << B << endl;
      else
        cout << ( A[ i ] > B[ i ] ? A : B ) << endl;
      break;
    }
  return 0;
}