Capacity of a class of deterministic relay channels

The capacity of a class of deterministic relay channels with transmitter input X, receiver output Y, relay output Y-1 = f(X, Y), and separate noiseless communication link of capacity R-0 from the relay to the receiver, is shown to be C(R-0) = sup(p(x)) min{I(X ; Y) + R-0, I(X ; Y, Y-1)}. Roughly speaking, every bit from the relay is worth one bit to the receiver until saturation at capacity.