The reference frame, in which the centre of inertia of a given system of particles is at

Question:

The reference frame, in which the centre of inertia of a given system of particles is at rest, translates with a velocity V relative to an inertial reference frame K. The mass of the system of particles equals m, and the total energy of the system in the frame of the centre of inertia is equal to E. Find the total energy E of this system of particles in the reference frame K.