Bipartite Graphs and Matchings · Easy problem

Four tasks $a,b,c,d$ must each run on a machine: $a$ can only use machine $1$, $b$ can only use machine $1$ as well, $c$ can only use machine $2$, and $d$ can use machine $2$ or machine $3$. What is the maximum number of tasks that can run at the same time, one machine per task?