foo.cc:2:16: error: expected identifier before ';' token
    2 | using namespace;
      |                ^
foo.cc: In function 'int main()':
foo.cc:6:2: error: 'cin' was not declared in this scope; did you mean 'std::cin'?
    6 |  cin>>x;
      |  ^~~
      |  std::cin
In file included from foo.cc:1:
/usr/include/c++/9/iostream:60:18: note: 'std::cin' declared here
   60 |   extern istream cin;  /// Linked to standard input
      |                  ^~~
foo.cc:7:2: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
    7 |  cout<<"x1+x2=x"<<endl;
      |  ^~~~
      |  std::cout
In file included from foo.cc:1:
/usr/include/c++/9/iostream:61:18: note: 'std::cout' declared here
   61 |   extern ostream cout;  /// Linked to standard output
      |                  ^~~~
foo.cc:7:19: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
    7 |  cout<<"x1+x2=x"<<endl;
      |                   ^~~~
      |                   std::endl
In file included from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/ostream:599:5: note: 'std::endl' declared here
  599 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~
foo.cc:5:11: warning: unused variable 'x1' [-Wunused-variable]
    5 |     int x,x1,x2;
      |           ^~
foo.cc:5:14: warning: unused variable 'x2' [-Wunused-variable]
    5 |     int x,x1,x2;
      |              ^~

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