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