The lazy boys

Alice and Bob, both have to drink water. But they both don't want to go, so they will play a game to decide who will fetch water for both of them. Alice will choose a number randomly between 1 and N (both inclusive) and Bob will choose a number randomly between 1 and M (both inclusive). Both will write their numbers on a slip of paper. If sum of numbers choosen by both is odd, then Alice will go