Two masses m are attached to opposite sides of a rigid rotating shaft in the vertical plane. Another pair of equal masses m₁ is attached to the opposite sides of the shaft in the vertical plane as shown in figure. Consider m = 1 kg, e = 50 mm, e₁ = 20 mm, b = 0.3 m, a = 2 m and a₁ = 2.5 m. For the system to be dynamically balanced, m₁ should be ______ kg.