Theorem numeration shift

Theorem numeration shift

I would like to numerate my theorems not by chapters but by shifted
chapters in such a way that the theorems in chapter 2 would read Theorem
1.1, Theorem 1.2 and so on.
I need something along the lines of
\newteorem{thm}{Theorem}["chapter - 1"]
where I don't know the correct syntax for "chapter - 1".