How to Response.Redirect to folder layer higher than the project folder?

For example: In the server (http://www.companyA.com/login.aspx)
/MyWebApp/frmMain.aspx
/MyWebApp/frmJob1.aspx
/MyWebApp/frmJob2.aspx
/AnotherWebApp/frmMain.aspx
/AnotherWebApp/frmJob1.aspx
/AnotherWebApp/frmJob2.aspx
login.aspx (1st page always)
 
After login from login.aspx, the next page loaded is /MyWebApp/frmMain.aspx. If I have a btnBACK in /MyWebApp/frmMain.aspx, how can I redirect is back to login.aspx without entering the full path?
I tried to do Response.Redirect("../login.aspx") but VWD 2008 will auto change it to Response.Redirect("login.aspx") so now I'm doing Response.Redirect("http://www.companyA.com/login.aspx") hardcoding the full path..

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