There are 2 trees in a garden (tree "A" and "B") and on the both trees are some birds. The birds of tree A say to the birds of tree B that if one of you comes to our tree, then our population will be the double of yours. Then the birds of tree B tell to the birds of tree A that if one of you comes here, then our population will be equal to that of yours. How many birds are there on A & B?