Combined header cell in a table

\documentclass{article}

\usepackage{picture,slashbox}

\begin{document}

\begin{tabular}{@{}c|*6c} 
\backslashbox[0pt][l]{}{}
  & \makebox(0,0)[lb]{\put(-1.25\tabcolsep,0){\llap{k}}}%
a & 0 & 1 & 2 & 3 \\\hline
\makebox(0,0){\put(0,2.25\normalbaselineskip){\rlap{n}}}%
0 & b & 1 & 2 & 3 & 4\\
1 & c & 1 & 2 & 3 & 4\\
2 & d & 1 & 2 & 3 & 4\\
\end{tabular}

\end{document}