Here are some sample runs of my version of this program:

john@vega:(~/CS1500) fence
I can make a fence with from 2 to 18 posts.
How many posts would you like to have in your fence? 1
Sorry, no can do.
john@vega:(~/CS1500) fence
I can make a fence with from 2 to 18 posts.
How many posts would you like to have in your fence? 0
Sorry, no can do.
john@vega:(~/CS1500) fence
I can make a fence with from 2 to 18 posts.
How many posts would you like to have in your fence? -23
Sorry, no can do.
john@vega:(~/CS1500) fence
I can make a fence with from 2 to 18 posts.
How many posts would you like to have in your fence? 2
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john@vega:(~/CS1500) fence
I can make a fence with from 2 to 18 posts.
How many posts would you like to have in your fence? 3
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john@vega:(~/CS1500) fence
I can make a fence with from 2 to 18 posts.
How many posts would you like to have in your fence? 12
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john@vega:(~/CS1500) fence
I can make a fence with from 2 to 18 posts.
How many posts would you like to have in your fence? 18
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john@vega:(~/CS1500) fence
I can make a fence with from 2 to 18 posts.
How many posts would you like to have in your fence? 19
Sorry, no can do.
john@vega:(~/CS1500) fence
I can make a fence with from 2 to 18 posts.
How many posts would you like to have in your fence? 20
Sorry, no can do.
john@vega:(~/CS1500)