Security :: How To Redirect To Specific Page Based On User Roles

I am using Formsauthentication. My situation is as per below:

the login form has codebehind :

protected void LoginButton_Click(object sender, EventArgs e)
{
TextBox uname = Login1.FindControl("UserName") as TextBox;
TextBox pass = Login1.FindControl("Password") as TextBox;
CheckBox rm = Login1.FindControl("RememberMe") as CheckBox;
Literal fail = Login1.FindControl("Literal1") as Literal;
if (Membership.ValidateUser(uname.Text, pass.Text))

[Code].....

Now the problem is that when I try to login with proper credentials it first redirects to default.aspx( there is no such page in my project), after login again with same credentials it properly redirects to the correct page. Why such problem arises? to fix this?