[UPSC CSE 2008] Statistical Physics (Q307, 10M)

Consider N independent particles, each of which can be in either of two energy states +\epsilon and -\epsilon. Derive an expression for the entropy of a microcanonical ensemble for this system, where $ f = \sum_i \epsilon_i / N\epsilon $ is fixed, where $ \epsilon_i $ is the energy of the i-th particle.