/**
         * @brief solving the math problem
         */
        private void Solve()
        {
            string problem = numberTextBox.Text;
            double result  = Parser.Solve(problem);

            numberTextBox.Text = double.IsNaN(result) ? "error" : result.ToString().Replace(',', '.');
            solved             = true;
            FocusManager.SetFocusedElement(this, numberTextBox);
        }