Main.cc:2:7: error: expected nested-name-specifier before 'namesprace' using namesprace std; ^ Main.cc: In function 'int mian()': Main.cc:6:5: error: 'cin' was not declared in this scope cin>>a>>b>>c; ^ Main.cc:6:5: note: suggested alternative: In file included from Main.cc:1:0: /usr/include/c++/5/iostream:60:18: note: 'std::cin' extern istream cin; /// Linked to standard input ^ Main.cc:7:5: error: 'cout' was not declared in this scope cout<& __os) ^