Start and run a staad model Command line

Hello,

I am trying to run a staad model with comman line, but I can`t do it

My folder instalation is "C:\SProV8i SS6\STAAD\Staadpro.exe"

And my staad model on c:\a\model.std

If I use   "C:\SProV8i SS6\STAAD\Staadpro.exe" c:\a\model.std 

The model opens

I have read on staad help that I have use the next sintasix

"C:\SProV8i SS6\STAAD\Staadpro.exe" STAAD c:\a\model.std /s

 and it doesn`t work

Thanks in advanced