evolution - Effect of sex-ratio on the effective population size

As stated on this wikipedia article, the effective population size $N_e$ when the sex ratio differs from $\frac{1}{2}$ is

$$N_e = \frac{4N_mN_f}{N_m+N_f}$$

I understand the biased sex-ratio decreases the effective population size and the formula is quite simple. However, I would fail to prove that this formula is right.

Can you demonstrate to me that $N_e = \frac{4N_mN_f}{N_m+N_f}$ is true?