Stacking Images in Beamer

Question

So I have this code that places images side by side in a frame:

\documentclass[]{beamer}
\usepackage[utf8]{inputenc}
\begin{document}
\begin{frame}[allowframebreaks]{Sample}
   \begin{figure}[ht]
        \begin{minipage}[b]{0.45\linewidth}
            \centering
            \includegraphics[width=\textwidth]{example-image-a}
            \caption{Label for a}
            \label{fig:a}
        \end{minipage}
        \hspace{0.5cm}
        \begin{minipage}[b]{0.45\linewidth}
            \centering
            \includegraphics[width=\textwidth]{example-image-b}
            \caption{Label for b}
            \label{fig:b}
        \end{minipage}
    \end{figure}
\end{frame}
\end{document}

How might one modify this to have the images stacked vertically instead of side by side?

Cheers.#