A train with cross-sectional area Sₜ is moving with speed vₜ inside a long tunnel of cross-sectional area S₀ (S₀ = 4Sₜ). Assume that almost all the air (density ρ) in front of the train flows back between its sides and the walls of the tunnel. Also, the air flow with respect to the train is steady and laminar. Take the ambient pressure and that inside the train to be p₀. If the pressure in the region between the sides of the train and the tunnel walls is p, then p₀ − p = [7/(2N)]ρvₜ². The value of N is ________.