Climbing ladders in simple platformer

I'm making a little game based upon a program from compute's programming atari book which is the yellow book and it's called Closeout I'm remaking it as a nes game and I can't figure out the climbing out - all he did is to load the zips to the project...so I'm screwed.

all the player is doing is jumping up XD by the ladder so I'm in need of help, so when I get this fixed I'll need to do an AI for the monsters to use the ladders as well and try to stop the player from getting the sale items.
 
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