wpf Datagrid: Auto Increment number column in datagrid

wpf Datagrid: Auto Increment number column in datagrid

Iam new to wpf , I have a datagrid with 5 columns ,now I want to set first
column as a serial number column ,column which has a sequence numbers
starting from 1 to n rows . If the user add or delete one row to the
datagrid the serial number must be auto update how to do this in wpf...?
My Xaml :
<my:DataGrid Name="dataGrid1" ItemsSource="{Binding}"
AutoGenerateColumns="False" Height="150"
BeginningEdit="dataGrid1_BeginningEdit" KeyDown="dataGrid1_KeyDown">
<my:DataGrid.Columns>
<my:DataGridTextColumn Header="Sl No"
Binding="{Binding}"></my:DataGridTextColumn>
<my:DataGridTextColumn Header="Product Code" Binding="{Binding
Product_Code}"></my:DataGridTextColumn>
<my:DataGridTextColumn Header="Product Name" Binding="{Binding
Product_Name}"></my:DataGridTextColumn>
<my:DataGridTextColumn Header="Purchase Rate" Binding="{Binding
PurchaseRate}"></my:DataGridTextColumn>
<my:DataGridTextColumn Header="Qty" Binding="{Binding
Qty}"></my:DataGridTextColumn>
<my:DataGridTextColumn Header="Amount" Binding="{Binding
Amount}"></my:DataGridTextColumn>
</my:DataGrid.Columns>
</my:DataGrid>
My code behind is:
public class clsItemCollection
{
public string ProductName { get; set; }
public decimal PurchaseRate { get; set; }
public int Qty { get; set; }
public decimal Amount { get; set; }
}
ObservableCollection<clsItemCollection> obsItems = new
ObservableCollection<clsItemCollection>();
dgvSales.ItemsSource = obsItems; //On window constructor