How to record a keypress

Hi

I am using Matlab to write an experiment code.

In this experiment, a number stream will be presented at the screen center (i.e. several numbers will be shown one by one).

This number stream will be shown for 4 s (each number for 0.2 s, with a 0.2-s break between two numbers), and then there is a big break (2 s).

I want to record the key press (either M or N, other key presses will be invalid and be ignored) while the numbers are being shown.

However, I don't know how to do this with Matlab PsychToolBox.

Can anyone please help me with this?

Thank you so much!