In constant sum game (zero sum game), if there are two parties then:

Constant Sum Game (Zero Sum Game)? A constant sum game is one in which the total pay-offs are the same for all possible combinations of players strategies. This means that each player can only gain at the expense of others; any players loss is balanced by an equal gain (or gains) made by the other players. A real life example of a constant sum game is gambling. If one player wins, other players must have lost the same amount. It is also called zero sum game b/c the sum of gains/losses of all players become zero .For example if there are two players then: Gains of one player + Losses of other player = 0}