operator+ in derived classes

This is something someone was asking in irc. I really don't need to do this
right now, but may have to in the future. The following code is in error
(marked).

#include <iostream>
#include <string>

class Base
{
public:
    Base( const int x = 0): x_( x ) {}
    Base operator+( const Base b ) { Base Temp( x_ ); Temp.x_ += b.x_;
return Temp; }
    int X() { return x_; }
    virtual ~Base() {}
private:
    int x_;
};

class Derived: public Base
{
public:
    Derived( const int x = 0, const int y = 0): Base( x ), y_( y ) {}
    int Y() { return y_; }
private:
    int y_;
};

int main()
{

    Base MyBase(10);
    std::cout << MyBase.X() << "\n";

    Base MyBase2 = MyBase + Base(5);
    std::cout << MyBase2.X() << "\n";

    Derived MyDerived( 10, 20 );
    std::cout << MyDerived.X() << " " << MyDerived.Y() << "\n";

    Derived MyDerived2 = MyDerived + Derived( 5, 10 );
    // Following line gives compilation error
    std::cout << MyDerived.X() << " " << MyDerived.Y() << "\n";
   // error C2440: 'initializing' : cannot convert from 'Base' to 'Derived'
   // No constructor could take the source type, or constructor
overload resolution was ambiguous

    std::string wait;
    std::getline( std::cin, wait );

}

I understand the error. I am tryign to operator+ on derived, but the only
operator + is on base.

How do people handle this?

What would be a nice solution, if possible, is to have a derived operator+
that calls the base operator+ for the addition of the x_'s, then add the
y_'s in derived operator+ and return derived. I couldn't come up with a way
to do this however, and think I would have to totally rewrite the operator+
for derived. In this case it's trivial, but I could see a complex class
where it wouldn't be so trivial.

Any thoughts?