Blender game engine (sequential animations by key pressure)

Hi guys!
I’m new in this game engine and i’m stuck…
I want to play animations sequentially step by step pressing a certain key of the keyboard, in this case the right arrow. I wanted go backwards by pressing the left arrow, always step by step.
So if I press the right arrow it plays the 0-20 animation, if i press it again it plays the 20-40 animation, but if I press the left arrow it plays the 40-20 animation…

I don’t know how to set the left arrow to play backwards the animation.