How do I Make my player only go up and down with arrow keys

How do I Make my player only go up and down with arrow keys

Example i have a player flying in the air and have objects fly at the player the player has to go up and down on arrow keys

Set Condition

if up arrow key is down then [action] Set (player.y-1)*dt
if down arrow key is down the [action] Set (player.y+1)*dt

can u make me a capx please

Here's the capx!
(After you press one of the buttons, press anywhere inside the screen to move the player.)