Get the elements in a vector that are not repeated a specified number of times

I have a vector that looks like this:

v <- c(1,2,2,3,3,3,4,4)

How can I get the elements that are not repeated twice? In other words, they are only present once or more than twice in the vector.

Thank you!

>Solution :

Try subset with ave

> subset(v, ave(v, v, FUN = length) != 2)
[1] 1 3 3 3