making a box in beamer — adding labels diagonally

Question

I would like to add a label that is diagonal to the following box, underneath the n', such that it is horizontally just below the box. Basically, I would like to "subtract" the difference between the content in the two cells. I'd like to add a label that says "Diff" just off the bottom left corner and then place the difference just below (p, n') box, outside of it.

For a picture of the box, see an earlier questions: making a box in beamer -- center aligning the title

Thank you very much for your help!

\documentclass[professionalfont, fleqn]{beamer}
\usepackage{tikz}
\usetikzlibrary{arrows,shapes,positioning,fit,shapes.misc}

\begin{document}

\begin{frame}

\centering
 \only<1>{%
 \begin{tikzpicture}[
 box/.style = {draw,rectangle,minimum size=1.0cm,text
width=1cm,align=center}
 ]
\matrix (conmat) [row sep=0cm,column sep=0cm,ampersand
replacement=\&] {
\node (tpos) [box,label=left:\( \mathbf{p'} \),label=above:\(
\mathbf{p} \),] {};
  \& \node (fneg) [box,label=above:\textbf{n},] {$\checkmark$}; \\
 \node (fpos) [box,label=left:\( \mathbf{n'} \),] {$\checkmark$};
  \& \node (tneg) [box] {$\checkmark$}; \\
  };

\node [left=.05cm of conmat,text width=1.5cm,align=center]
{\textbf{actual \\ value}};
\node [above=.05cm of conmat] {\textbf{prediction}};
\end{tikzpicture}%
}

\end{frame}
\end{document}

Answer 1

I think beamer part is not related to this and previous question as it is only using TikZ content for just one frame. Just addding \node[below left= 2mm of conmat.south west] {diff}; to my answer given in the previous question linked is a solution.

\documentclass{beamer}
\usefonttheme{professionalfonts}
\usepackage{tikz}
\usetikzlibrary{positioning,matrix}
\begin{document}

\begin{frame}

\centering
\only<1>{%
 \begin{tikzpicture}
\matrix (conmat) [
draw,
matrix of math nodes, 
nodes in empty cells,
ampersand replacement=\&,
nodes={minimum size=1cm,outer sep=0,inner sep=0,anchor=center}
] {
                     \& \checkmark \\
\checkmark           \& \checkmark \\
  };
\draw (conmat.north) -- (conmat.south) (conmat.east) -- (conmat.west);
\node[above=2mm of conmat-1-1] {\(\mathbf{p}\)};
\node[left =2mm of conmat-1-1] {\(\mathbf{p'}\)}; 
\node[left =2mm of conmat-2-1] {\(\mathbf{p'}\)};
\node[above=2mm of conmat-1-2] {\textbf{n}};

\node[below left= 2mm of conmat.south west] {diff};

\node [left= 4mm of conmat,text width=1.5cm,align=center] {\textbf{actual \\ value}};
\node [above=4mm of conmat] {\textbf{prediction}};
\end{tikzpicture}%
}
\end{frame}
\end{document}