Bipartite Graphs and Matchings · Medium problem

A hiring manager has $4$ candidates and $4$ open roles. Candidate $1$ is qualified only for role $a$; candidate $2$ only for role $a$ as well; candidate $3$ for roles $a$ or $b$; candidate $4$ for roles $b$, $c$, or $d$. What is the maximum number of candidates who can be simultaneously placed into distinct roles?