onclick+postbackurl

Question

say  i have this code

for first time it validate  if  checkbox=true but if i uncheck  then i have to  push button twise for post back  why . 

 

 

protected void Button1_Click(object sender, EventArgs e)

{

 

if ((Chk1.Checked==true))

{

Response.Write("Please check");

}

else

Button1.PostBackUrl = "Detail.aspx";

}