Inheriting protected constructor

Question

As far as I understood, the using keyword can be used for inheriting constructors of a base class. In this code I'm trying to reuse the constructors of Base while making them public. Compilers say error : calling a protected constructor of class 'Base'. What is it that I'm misunderstanding here?

struct Base
{
protected:
    Base() = default;

    Base(int, int) {}
};

struct Derived1 : public Base
{
public:
    using Base::Base;

};

//...

Derived1 d2(4,2); // ERROR