Mathematical term for "opposite" in this context? A friend wanted to play a game where we would roll a die, and if it landed even then the wish would come true, but if it landed odd then the opposite of the wish would come true. So let's say I have five dollars and my wish is to gain five dollars. 

I know the negation of gaining five dollars is not gaining five dollars. But that is not the 'opposite' of my wish in this context. The opposite would be losing five dollars. What is the operation called that's being applied to "gaining five dollars" such that it becomes "losing five dollars"?
Change the rules of the game. The game costs \$5 to play and you can gain $10 by winning.
Change the rules of the game. The game costs \$5 to play and you can gain $10 by winning.