如何指定节点之间的距离？

Question

我想指定一个距离right of：

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{positioning}
\usetikzlibrary{shapes,arrows,shadows,decorations.markings}
\begin{document}
\tikzstyle{block} = [rectangle, draw, 
    text  width=5em, text centered, rounded corners, minimum height=2em]
\tikzstyle{rq} = [block, text  width=12em]
\tikzstyle{line} = [draw, -latex']

\begin{figure}
\begin{tikzpicture}[node distance=2cm, auto]
 \node [block] (f) {A1. First};

 \node[block, below of = f] (s) {A2. Second};

 \node[rq, right = 5cm of=f] (r1) {RQ1. Some\\ RQ2. Questions};

\path [line] (f) -- node {} (r1);

\end{tikzpicture}
\end{figure}

\end{document}

但我明白

程序包 PGF 数学错误未知运算o' or符'

好像我没有positioning正确加载！？