Shortest Possible Project Completion Time

A project has six tasks with durations (in hours): A=3, B=2, C=4, D=1, E=5, F=2. The precedence constraints are: A must finish before C and D start; B must finish before D starts; C and D must both finish before E starts; D must finish before F starts. Any number of tasks may run in parallel as long as all their prerequisites are complete. Starting at time 0, what is the earliest time at which all six tasks are finished?