Bipartite Graphs and Matchings · Medium problem

Six requests $1,2,\dots,6$ come in for time slots: $1$ and $2$ need slot $a$ only; $3$ and $4$ can each use slot $a$ or slot $b$; $5$ needs slot $c$ only; $6$ needs slot $d$ only. What is the maximum number of requests that can be scheduled into distinct slots at once?