private void button1_Click(object sender, EventArgs e)
        {
            ClBook b = new ClBook(Author, Title, PublishHouse,
                                  Page, Year, InvNumber, Existence);

            if (ReturnTime)
            {
                b.ReturnSrok();
            }
            b.PriceBook(PeriodUse);
            its.Add(b);
            Author    = Title = PublishHouse = "";
            Page      = InvNumber = PeriodUse = 1;
            Year      = 1;
            Existence = ReturnTime = false;
        }
        private void button1_Click_1(object sender, EventArgs e)
        {
            ClBook b = new ClBook(textBox1.Text, textBox3.Text, textBox2.Text, (int)numericUpDown4.Value, (int)numericUpDown3.Value, (int)numericUpDown2.Value, checkBox1.Checked);

            if (ReturnTime)
            {
                b.ReturnSrok();
            }
            b.PriceBook(PeriodUse);
            its.Add(b);
            Author    = Title = PublishHouse = "";
            Page      = InvNumber = PeriodUse = 1;
            Year      = 1;
            Existence = ReturnTime = false;

            listBox1.Items.Add(b.ToString());
        }