How to make the behavior of the enemy?

Guys! Tell me, please, how to make the enemy begin to pursue the player if the player approaches him at a certain distance, but if the player leaves the enemy's zone of action, he ceases to pursue the player. Please, show a screenshot of this scheme, if it's not a secret. Thank you.

http://www.stencyl.com/blocks/view/event-actors
http://www.stencyl.com/blocks/view/number-trig
http://www.stencyl.com/blocks/view/scene-regions
You find

(Pac-man? All people skip you-)

thnx