#include <iostream>
#include <fstream>

using namespace std;

int main(int argc, char* argv[])
{
  if (argc != 3)
  {
    cerr << "ERROR: You must specify exactly two files on the command line."
         << endl;
    return 1;
  }

  ifstream f1( argv[1] );

  if ( ! f1 )
  {
    cerr << "ERROR: '" << argv[1] << "' could not be opened." << endl;
    return 1;
  }
  
  ifstream f2( argv[2] );
  
  if ( ! f2 )
  {
    cerr << "ERROR: '" << argv[1] << "' could not be opened." << endl;
    f1.close();
    return 1;
  }

  // We only need two integer memory space to solve the problem,
  // so we can handle files many times larger than internal memory
  // (Filling a vector will be bad if files are 1000TB large.)
  int i1, i2;
  
  // Read first two from each file to know where to start
  f1 >> i1;
  f2 >> i2;

  // Stop reading once one file reach the end
  while ( f1 && f2 )
  {
    if ( i1 < i2 )
    {
      cout << i1 << endl;
      f1 >> i1;
    }
    else
    {
      cout << i2 << endl;
      f2 >> i2;
    }
  }

  // Dump the remaining items from the larger file
  while ( f1 )
  {
    cout << i1 << endl;
    f1 >> i1;
  }
  
  while ( f2 )
  {
    cout << i2 << endl;
    f2 >> i2;
  }
  
  f2.close();
  return 0;
}