Tikz grid in an environment

Question

I would like to create an environment having the following role: create an exercise (called "exo" in the sequel) and a grid with a prescribed number of lines. My environment exo works fine, but not with the grid. I tried

\newenvironment{exoQuadrill}[1]%
{\begin{exo}}%
{\end{exo} \smallbreak\begin{tikzpicture}[scale=0.5,gray]
\draw(0,0) grid (33.4,#1);
\end{tikzpicture}\medbreak}

It doesn't work (error : "illegal parameter number"), but it is if I don't specify the number of lines:

\newenvironment{exoQuadrill}[1]%
{\begin{exo}}%
{\end{exo} \smallbreak\begin{tikzpicture}[scale=0.5,gray]
\draw(0,0) grid (33.4,13);
\end{tikzpicture}\medbreak}