The design specifications of a 1.2-m-long solid circular transmission shaft require that the angle of twist of the shaft not exceed 4 when a torque of 810 N-m is applied. Determine the required diameter of the shaft, knowing that the shaft is made of a steel with an allowable shearing stress of 90 MPa and a modulus of rigidity of 77.2 GPa. The required diameter of the shaft is, mm.