Four hostage mathematicians have been burried up to their necks (as above) such that they
can't turn their heads! Their captors say that they will be set free if someone can say what
colour their own hat is. They can't say anything but the correct answer or they get killed.
Each guy only knows the colour of the hat(s) he can see and that of the 4 hats, 2 are red and 2 are blue.
Nobody can see through the wall between the 3rd and 4th hostages.
How do they get the right answer with 100% certainty and escape alive?!