In a survey of $N = 1000$ university students, a researcher uses stratified sampling. The population is divided into $N_1 = 400$ (Freshmen), $N_2 = 300$ (Sophomores), $N_3 = 200$ (Juniors), and $N_4 = 100$ (Seniors). If the researcher selects a sample size $n = 100$ using proportional allocation, how many Juniors ($n_3$) should be included?