Fence post activity

During the fence post activity, you will end up with 7 posts.  For every two yards, you have a fence post.  There is 12 yards, therefore 12/2=6 posts.  These 6 posts do not include the last one.  You need to add one fence post to the end to finish of the fence.  If "N" represents number of posts, the expression is
# of posts= 2N + 1