value of confirm msgbox(javascript) in code behind(aspx.cs)page?

<form runat="server">
       <asp:button id="btn1" runat="server" onclientclick="confirming();" onclick="btn1_Click" text="Submit" xmlns:asp="#unknown" />
 </form>
   <script type="text/javascript">
       function confirming() {
           document.getElementById('<%=confirmValue.ClientID%>').value = confirm('Yes or no?');
       }
   </script>

protected void btn1_Click(object sender, EventArgs e)
      {
          string confirmValueStr = confirmValue.Value;
          bool confirmValueBool;
          if (bool.TryParse(confirmValueStr, out confirmValueBool)
          {
              // now, confirmValueBool is the value of the confirm function
          }
          else
          {
              // can't convert confirmValueStr to confirmValueBool
          }
      }