/in/foo.cc:2:7: error: expected nested-name-specifier before 'namespaces'
 using namespaces std;
       ^~~~~~~~~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:4:2: error: 'cin' was not declared in this scope
  cin>>a>>b;
  ^~~
/in/foo.cc:4:2: 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:4:7: error: 'a' was not declared in this scope
  cin>>a>>b;
       ^
/in/foo.cc:4:10: error: 'b' was not declared in this scope
  cin>>a>>b;
          ^
/in/foo.cc:5:2: error: 'cout' was not declared in this scope
  cout<<a+b;
  ^~~~
/in/foo.cc:5:2: 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
                  ^~~~