Why using directive behaves differently in global scope and in local scope?

When I write following code it gets compiled and executed properly:

#include <iostream>
using namespace std;

namespace first
{
  int x = 5;
  int y = 10;
}

namespace second
{
  double x = 3.1416;
  double y = 2.7183;
}

int main () {
  using namespace first; //using derective
  using second::y;
  cout << x << endl;
  cout << y << endl;
  return 0;
}

But if I write using directives outside main function as follows,

using namespace first; //using derective
using second::y;
int main () {
  cout << x << endl;
  cout << y << endl;
  return 0;
}

It gives this compilation error:

g++     namespace03.cpp   -o namespace03
namespace03.cpp: In function ‘int main()’:
namespace03.cpp:20:11: error: reference to ‘y’ is ambiguous
namespace03.cpp:13:10: error: candidates are: double second::y
namespace03.cpp:7:7: error:                 int first::y
make: *** [namespace03] Error 1

Can anybody explain why using directive behaves differently when it is used inside main and outside main?