private void Savebtn_Click(object sender, EventArgs e)
        {
            if(ProductQTYTb.Text=="" || Convert.ToInt32(ProductQTYTb.Text) > Stock)
            {
                MessageBox.Show("Insufficient Stock");
            }else if (ProductQTYTb.Text == "" || Key == 0)
            {
                MessageBox.Show("Missing information");
            }
            else
            {
                int total= Convert.ToInt32(ProductQTYTb.Text)*Convert.ToInt32(ProductMFeeTb.Text);
                DataGridViewRow newRow = new DataGridViewRow();
                newRow.CreateCells(BillDGV);
                newRow.Cells[0].Value = n+1;
                newRow.Cells[1].Value = ProductNameTb.Text;
                newRow.Cells[2].Value= ProductQTYTb.Text;
                newRow.Cells[3].Value= ProductMFeeTb.Text;
                newRow.Cells[4].Value = total;
                GrdTotal += total;
                BillDGV.Rows.Add(newRow);
                n++;
                TotalLbl.Text = "£" + GrdTotal;
                UpdateStock();
                Reset();
            }
        }