Marking down an arrow in an equation

How to write this equation in LaTex?

$$y_t^s=\underset{\underset{\scriptstyle\text{Output of firms with one year old contracts}{\scriptstyle\downarrow}}{\frac{1}{2}(P_t-w_t(t-1)+u_t)}+\underset{\underset{\scriptstyle\text{Output of firms with two year old contracts}{\scriptstyle\downarrow}}{\frac{1}{2}(P_t-w_t(t-2)+u_t)}$$

I wrote this, but couldn't get it.  \documentclass[border = 1cm]{standalone}
\usepackage{amsmath} % For \text{...}
\begin{document}
{$\arraycolsep=2ex \begin{array}{ccc} \dfrac{1}{2}[P_t - w_t(t-1) + u_t] & + & \dfrac{1}{2}[P_t-w_t(t-2)+u_t]\\ \Bigg\downarrow & & \Bigg\downarrow\\ \text{\scriptsize Output of firms with workers} & & \text{\scriptsize Output of firms with workers}\\ \text{\scriptsize on one-year old contract} & & \text{\scriptsize on two-year (expiring) contract}\\ \end{array}$}
\end{document}

This is my proposal

\documentclass{article}
\usepackage{amsmath}
\begin{document}
$y_t=\underbrace{\frac{1}{2}[P_t-w_t(t-1)+u_t]} _{\substack{\text{Output of firms with workers}\\\text{on one-year old contract}}}+ \underbrace{\frac{1}{2}[P_t-w_t(t-2)+u_t]} _{\substack{\text{Output of firms with workers}\\\text{on two-year (expiring) contract}}}$
\end{document} A variant with some improvements – in particular medium-size fractions from nccmath and the normal spacing for the + sign:

\documentclass{article}
\usepackage{mathtools, nccmath}

\begin{document}

$y_t=\underset{\mathclap{\substack{ \rule{0.4pt}{2em}\\ \strut\text{Output of firms with workers}\\\text{on one-year old contract}}}}{\mfrac{1}{2}\bigl[P_t-w_t(t-1)+u_t\bigr]} + \underset{\mathclap{\substack{\rule{0.4pt}{2em}\\ \strut\text{Output of firms with workers}\\\text{on two-year (expiring) contract}}}}{\mfrac{1}{2}\bigl[P_t-w_t(t-2)+u_t\bigr]}$

\end{document} EDIT:

My proposal with the tag is this: \documentclass[a4paper,12pt]{article}
\usepackage{mathtools,graphicx}
\begin{document}
$y_t=\underset{\underset{\substack{\text{Output of firms with workers} \\ \text{on one-year old contract}}}{\rotatebox{-90}{\xrightarrow{\makebox[1cm]{}}}}}{\dfrac{1}{2}[P_t - w_t(t-1) + u_t]}+\underset{\underset{\substack{\text{Output of firms with workers} \\ \text{on two-year (expiring) contract}}}{\rotatebox{-90}{\xrightarrow{\makebox[1cm]{}}}}}{\dfrac{1}{2}[P_t-w_t(t-2)+u_t]}$
\end{document}

or you could use also tikz-cd, used to typeset commutative diagrams and also for other things.... \documentclass[a4paper,12pt]{article}
\usepackage{tikz-cd,mathtools}
\begin{document}
\begin{tikzcd}[column sep=-.4cm]
{y_t=\dfrac{1}{2}[P_t - w_t(t-1) + u_t]}+ \arrow[d] & {\dfrac{1}{2}[P_t-w_t(t-2)+u_t]} \arrow[d] \\
\substack{\text{Output of firms with workers} \\ \text{on one-year old contract}} & \substack{\text{Output of firms with workers} \\ \text{on two-year (expiring) contract}}
\end{tikzcd}
\end{document}
cument}