#include <iostream>
using namespace std;

class Base
{
	public:
		virtual void Foo() = 0;
};

class Derived
	: public Base
{
	public:
		// void Foo() { } // Derived cannot be instantiated, it MUST define Foo()
		                  // OR its derived classes MUST define Foo(). The 
		                  // method is NOT optional when you want to instantiate
		                  // the class
};

int main() {
	// your code goes here
	Derived d;
	return 0;
}