A spring - block system is resting on a frictionless floor as shown in the figure. The spring constant is $2.0 \mathrm{N}\mathrm{ }{\mathrm{m}}^{-1}$ and the mass of the block is $2.0\mathrm{kg}$ . Ignore the mass of the spring. Initially the spring is in an unstretched condition. Another block of mass $1.0\mathrm{kg}$ moving with a speed of $2.0\mathrm{m}\mathrm{ }{\mathrm{s}}^{-1}$ collides elastically with the first block. The collision is such that the $2.0\mathrm{kg}$ block does not hit the wall. The distance, in metres, between the two blocks when the spring returns to its unstretched position for the first time after the collision is _________.