Vbulletin 4 SHOWTHREAD

Hello,
I would like to include in the <title> tag also the [forum_title].At the moment I have in the SHOWTHREAD the following :
<title> {vb:raw thread.prefix_plain_html} {vb:raw thread.title} </title>

I would like to have (example for this post on vbseo.com )

<title> Vbulletin 4 SHOWTHREAD - General Discussion </title>

What syntax I need to use on vb4 to get the forum title for this thread?