The correct answer should be always 0, :)

If zero is the number that's input into the function, you end up with -1 bottles of beer on the wall?

Negative beer amounts? Oh dear

Negative amounts of beer? Means you are drunk then:)

Do you seriously require a matlab program to solve this?:/

Problem worth solving.
Beer all the way..Cheers..!!

could be a bit more challenging...
For instance, why don't calculate the statistic probabillity how many beer is drunk by male & female party people during one night. With exponential increasing amount of drunken beer in the later hours... ;-)