private void calculateButton_Click(object sender, EventArgs e)
        {
            var inputModel = new TaxInputModel
            {
                MonthlySalary = salaryUpDown.Value,
                NrOfChildren = (int)childrenUpDown.Value
            };

            var calculator = new TaxCalculator();

            var result = calculator.Calculate(inputModel);

            var resultBuilder = new StringBuilder();
            resultBuilder.AppendFormat("Net salary: {0} Ft", result.NetSalary);
            resultBuilder.AppendLine();
            resultBuilder.AppendFormat("Total cost: {0} Ft", result.TotalCost);
            resultBuilder.AppendLine();

            resultTextBox.Text = resultBuilder.ToString();
        }