Number of arguments in operator overload in C++

I'm learning C++ and I created two simple hello-world applications. In both of them I use operator overload, but here is the problem. On the first one, I can provide two arguments to overload operator, and it's fine.

Header:

enum Element {a,b,c,d,e};
Element operator + (Element x, Element y);
//more overloads for -, *, / here

Source:

Element operator + (Element x, Element y) {
    return ArrayOfElements[x][y];
}

But in my second app (simple complex number calculator) - this method didn't work. After googling and figuring out why, I end up with this code:

Header:

struct Complex {
        double Re;
        double Im;

        Complex (double R, double I) : Re(R), Im(I) { }

        Complex operator + (Complex &Number);
        //more overloads
    };

Source:

Complex Complex::operator + (Complex &Number)
    {
        Complex tmp = Complex(0, 0);
        tmp.Re = Re + Number.Re;
        tmp.Im = Im + Number.Im;
        return tmp;
    }

It's working now, but I want to know, why in the first piece of code I was allowed to put two arguments in operator overloading, but with the second one I was given the following error?

complex.cpp:5:51: error: 'Complex Complex::operator+(Complex, Complex)' must take either zero or one argument

It's the same whenever I use classes or not. I've been seeking through many docs and the second way seem to be more correct. Maybe it's because of different argument types?

Both sources compiled with -Wall -pedantic parameters using g++, both are using the same libraries.