Getting error on row command of GridView

i m getting an error(Input string was not in a correct format.) on click of detail button please help me to resolve this problem.

 protected void grdexam_RowCommand(object sender, GridViewCommandEventArgs e)

    {

        if (e.CommandName == "dtls") 

        {

            Button btn = (Button)e.CommandSource;

            var index =Convert.ToInt32(btn.CommandArgument.ToString());

            GridViewRow row = grdexam.Rows[index];

            Label l1 = (Label)row.FindControl("Label1");

            SqlCommand cmd = new SqlCommand("select * from ExamMaster where groupid='" + Session["grpid"] + "' and papercode='" + l1 + "'", con);

            SqlDataAdapter adp = new SqlDataAdapter();

            DataTable dt = new DataTable();

            adp.Fill(dt);

            if (dt.Rows.Count > 0)

            {

                GridView1.DataSource = dt;

                GridView1.DataBind();

            }

            else 

            {

                Response.Write("wromg");

            }

        }

    }

please help me

thanks in advance