/in/foo.cc: In function 'int main()':
/in/foo.cc:12:3: error: 'cin' was not declared in this scope
   cin>>x1>>y1>>x2>>y2>>x3>>y3;
   ^~~
/in/foo.cc:12:3: note: suggested alternative:
In file included from /in/foo.cc:2:0:
/usr/include/c++/6/iostream:60:18: note:   'std::cin'
   extern istream cin;  /// Linked to standard input
                  ^~~