Bipartite Graphs and Matchings · Superchallenge problem

$X=\{1,2,3,4,5\}$ and $Y=\{a,b,c,d,e\}$ are joined by the edges $1a,1b,2a,2b,2c,3c,3d,4d,4e,5e$. How many of these $10$ edges belong to EVERY maximum matching of this graph (not just some maximum matching)?