Operator overloading template parameter

So I have a little class that implements a matrix. Everything works well, except whatever gave me the reason to post here. I've explained more about the problem in the actual code, using comments. Thanks in advance to anyone that can help! This is not the entire program, but it is big enough so that it can compile on its own.

#include <iostream>
#include <initializer_list>

template<class type_t, unsigned Rows, unsigned Columns>
class matrix
{
private:
    std::initializer_list<std::initializer_list<type_t>> elements;

public:
    type_t contents[Rows][Columns];

    matrix() {}

    matrix(const std::initializer_list<std::initializer_list<type_t>> &container)
        : elements(container)
    {
        unsigned i = 0;
        for (const auto &el : elements)
        {
            unsigned j = 0;
            for (const auto &num : el)
            {
                contents[i][j] = num;
                j++;
            }
            i++;
        }
    }

    unsigned rows() { return Rows; }
    unsigned columns() { return Columns; }

    type_t &operator()(const unsigned &i, const unsigned &j)
    {
        return contents[i][j];
    }

    template<unsigned rws, unsigned cls>
    auto operator*(matrix<type_t, rws, cls> &mat)
    {
        matrix<type_t, Rows, 3> ret_mat;  //OK, but only in case the return matrix has 3 columns
    matrix<type_t, Rows, mat.columns()> ret_mat; //Error. This is the desired result
                                //The error message tells me it needs to be a compile-time constant
                                //it's pretty obvious why the first works and what the problem is
                                //but i still have no idea how to get past this

        for (unsigned i = 0; i < this->rows(); ++i)
        {
            for (unsigned j = 0; j < mat.columns(); ++j)
            {
                for (unsigned in = 0; in < 2; ++in)
                    ret_mat.contents[i][j] += this->contents[i][in] * mat.contents[in][j];
            }
        }

        return ret_mat;
    }
};

int main()
{
    matrix<int, 4, 2> mat = { { 7, 3 },{ 2, 5 },{ 6, 8 },{ 9, 0 } };
    matrix<int, 2, 3> mat2 = { { 7, 4, 9 },{ 8, 1, 5 } };

    auto mat3 = mat * mat2;

    for (unsigned i = 0; i < mat3.rows(); ++i)
    {
        for (unsigned j = 0; j < mat3.columns(); ++j)
            std::cout << mat3(i, j) << " ";
        std::cout << std::endl;
    }
    std::cin.get();
}