Negative Standard Deviation? about, question, possible, excel

smartpants

Ok heres a question i ws thinking about when doing some work in excel...

The variance of a 2 asset port = (Wa^2)(Var a) + (Wb^2)(Var b) + 2(Wa)(Wb)(Std a)(Std b)(Correl (a,b))

My question is this...lets say the correlation is -1. Wouldnt it be possible to get a variance of the portfolio that is negative? What would that mean? or am i thinking a bout this wrong?