In a survey of $N=2000$ employees, the population is divided into two strata: $N_1=1000$ (Management) and $N_2=1000$ (Staff). A researcher uses Neyman allocation to allocate a total sample size $n=200$. If the population variances are $\sigma_1^2=100$ and $\sigma_2^2=400$, what is the sample size $n_2$ for the Staff stratum?