Bipartite Graphs and Matchings · Easy problem

Three students, $P$, $Q$, and $R$, need to be scheduled into time slots. $P$ can only attend slot $1$ or slot $2$. $Q$ can only attend slot $1$ or slot $2$ as well. $R$ can only attend slot $3$ or slot $4$. What is the maximum number of students who can be scheduled into distinct slots at the same time?