two farmers riddle

this is pretty easy, and it's kind of not even a riddle, but for some reason people seem to have trouble with it. We'll see, here goes

Farmer A has some trees and farmer B has some trees. Farmer A says to Farmer B, "Give me one of your trees and I shall have twice as many as you." Farmer B then says, "How about you give me one of your trees and we shall have the same number."

How many trees did each farmer have. (note: this also works with small children and cookies, rabbits and carrots, or old ladies and knitting needles, etc...lol)