Cant able to logout after session time out in C#.net applica

Question

I want to logout my application if user not use apllication till 20 mins, so itried like below but its not working..session going time out but not redirecting to login page.

 

<authentication mode="Forms">       

<forms defaultUrl="~/login.aspx" loginUrl="~/login.aspx" slidingExpiration="true" timeout="10"></forms>     

</authentication>