Two shafts are connected by a flange coupling that transmits a torque of 25 N·m. The flanges are fastened by four bolts of the same material on a radius of 30 mm. Find the required core diameter of the bolts (answer in mm) if the allowable shear stress for the bolt material is 30 MPa; then choose the standard bolt (core diameters: M3 2.387 mm, M3.5 2.764 mm, M4 3.141 mm).