Access to all checked radgridview 's rows via gridviewcheckboxcolumn (telerik component)

i want to access all checked row in RadGridView(Telerik component) via GridViewCheckBoxColumn. Note : User select checkbox at run time ... 

Screenshot

<pre> 

string s1 = "";

for(int i =0; i <= radGridView1.Rows.Count-1 ; i++ )
{
if(radGridView1.Rows[i].Cells[0].Value.ToString() !="true" && radGridView1.Rows[i].Cells[3].Value.ToString() != null && radGridView1.Rows[i].Cells[3].Value.ToString() != "")
{
s1+=","+ radGridView1.Rows[i].Cells[3].Value.ToString();
}
}