Vector whose elements add up to a value in R

I'm trying to create a vector whose elements add up to a specific number. For example, let's say I want to create a vector with 4 elements, and they must add up to 20, so its elements could be 6, 6, 4, 4 or 2, 5, 7, 6, whatever. I tried to run some lines using sample() and seq() but I cannot do it.

Any help appreciated.