Parity is a trick taking game using the standard deck of cards. You get the number of points equal to the number of tricks you take. However, if the number of trick is odd, then your score is negative. Thus, for 0,1,2,3,4,5,... tricks, you get 0,-1,2,-3,4,-5,... points, respectively. Since there are 13 tricks, someone must have a negative score!