Warning and errors of implicitly-defined namespace

Compilation error I receive is:
main.cpp:1:17: warning: using directive refers to implicitly-defined namespace 'std'
using namespace std;
^
main.cpp:11:5: error: use of undeclared identifier 'cout'
cout<<"Enter a whole number: \n";
^
main.cpp:12:5: error: use of undeclared identifier 'cin'
cin>>num1;
^
main.cpp:13:5: error: use of undeclared identifier 'cout'
cout<<"Enter another whole number: \n";
^
main.cpp:14:5: error: use of undeclared identifier 'cin'
cin>>num2;