[Tex/LaTex] Arrows with different directions

arrows

I want to drawing two arrows which different directions from 1 edge to 2 edge.

I want to print a and b on the arrows respectively.

enter image description here

Best Answer

mwe

\documentclass{article}
\usepackage{amssymb}
\usepackage{stackrel}
\begin{document}
$1\stackrel[b]{a}{\rightleftarrows}2$ 
\end{document}