Distribution of the baseballseries

In a baseball series played by two equally skilled local teams A and B in the small town of Champion; the series winner is decided as soon as one of the teams wins four games and it becomes clear that the other team’s share of winning games cannot beat the competing teams share of winning games out of the total number of games p;ayed in the series. Assume the results of different games played in the series are independent of each other. Find the p.m.f. of the distribution of N := duration” of the baseballseries.