How much karma is required for one to get the downvote buttons? Also, in the past there have been times that I have accidentally clicked the upvote button in a

How much karma is required for one to get the downvote buttons? Also, in the past there have been times that I have accidentally clicked the upvote button in an attempt to re-activate my browser window, and found that there was no way for me to undo that, whereas on reddit if I hit the upvote button again it cancels out!