private void btBack_Click(object sender, EventArgs e)
        {
            Form5_Expenses f = Form5_Expenses.GetInstance;

            f.Show();
            this.Hide();
        }
        private void btNext_Click(object sender, EventArgs e)
        {
            //save data for pdf file
            UserInputs_StatementOfSalary();
            Form5_Expenses f = Form5_Expenses.GetInstance;

            f.Show();
            this.Hide();     //Hide curent window form
        }