I'm currently going over some undergraduate exams on particle physics and I'm having problems with a specific reaction, namely
$$ \pi^- + p \to K^- + \Sigma^+ $$
which, in my opinion, is not allowed due to strangeness violation ($0 \rightarrow -2$), so it could only be a weak interaction. Since it's purely hadronic, though, it should be a strong interaction. Can anyone explain to me where I'm wrong? Or is the professor's answer wrong? He marked the "not allowed" answer with "wrong" (no further explanation).