Special Chess-Board

Goku and vegeta are fighting rigorously from a long time, but they can't arrive to a conclusion that who is better and stronger than the other. So they decided to do it by other means. Both of them started playing chess on an $N × M$ chess-board and vegeta has already placed $R$ Black rooks on the chess-board and then he asks goku to place one White rook on the chess-board such that it is safe .