How to create a QWidget that looks the same as the image below?

border

The widget should have just these kinds of borders, I don’t even know how to start it on the QPaintEvent

I mean how to calculate this ? this looks almost impossible to me, please any help

class Widget : public QWidget
{
    Q_OBJECT
public:

    Widget() 
    {
        setStyleSheet("background-color: transparent");
        setAttribute(Qt::WA_TranslucentBackground);
        setWindowFlags(Qt::FramelessWindowHint);
    };


    void paintEvent(QPaintEvent* event) override
    {
        QPainter painter(this);
        painter.setRenderHint(QPainter::Antialiasing);        

        const int borderWidth = 10;
        const int cornerRadius = 20;
        const QColor borderColor(255, 0, 0);

        QPainterPath path;
    }
};

int main(int argc, char* argv[])
{
   QApplication a(argc, argv);
   Widget* w = new Widget;
   w->show();
   return a.exec();
}