A, B, C, D, E and F are six robots connected by directed contact rules. What is the minimum number of robots required between robot A and robot B to send a message from A to B?

Each robot can contact some other robots, though none of the robots can contact all the robots. The rules of contacting are as follows:

• B can contact E and can be contacted by C.
• D can be contacted by A, E and F.
• F can contact C.
• D can contact C and E.
• F can be contacted by A.

A robot can receive a message from other robots that can contact it and can relay the message to another robot it is allowed to contact.