guys, why MyInt is abstract?

class INumber
{
public:
    virtual INumber & operator*(const INumber & other) const = 0;
};

class MyInt : public INumber
{
public:
    MyInt(int value) : value(value) {}

    virtual MyInt mult(const INumber & other) const
    {
        const MyInt & other_tmp = (const MyInt &)other;

        return value + other_tmp.value;
    }
    
    int value;
};