/in/foo.cc:3:7: error: expected nested-name-specifier before 'nnamespace'
 using nnamespace std;
       ^~~~~~~~~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:6:2: error: 'cin' was not declared in this scope
  cin>>a>>b;
  ^~~
/in/foo.cc:6:2: note: suggested alternative:
In file included from /in/foo.cc:1:0:
/usr/include/c++/6/iostream:60:18: note:   'std::cin'
   extern istream cin;  /// Linked to standard input
                  ^~~
/in/foo.cc:7:2: error: 'cout' was not declared in this scope
  cout<<a+b<<endl<<a-b<<endl<<a*b<<endl<<a/b<<endl<<static_cast<int>(pow(a,b))<<endl<<a%b<<endl;
  ^~~~
/in/foo.cc:7:2: note: suggested alternative:
In file included from /in/foo.cc:1:0:
/usr/include/c++/6/iostream:61:18: note:   'std::cout'
   extern ostream cout;  /// Linked to standard output
                  ^~~~
/in/foo.cc:7:13: error: 'endl' was not declared in this scope
  cout<<a+b<<endl<<a-b<<endl<<a*b<<endl<<a/b<<endl<<static_cast<int>(pow(a,b))<<endl<<a%b<<endl;
             ^~~~
/in/foo.cc:7:13: note: suggested alternative:
In file included from /usr/include/c++/6/iostream:39:0,
                 from /in/foo.cc:1:
/usr/include/c++/6/ostream:590:5: note:   'std::endl'
     endl(basic_ostream<_CharT, _Traits>& __os)
     ^~~~